This is, I believe, what is called 'crackfic'. Because I was thinking about the most common Lucy pairings and I somehow thought of this and couldn't resist. Please forgive me, Lucy.
(Also, it's not very edited, so forgive me, readers.)
It all began innocently enough. Natsu and Happy met this girl, this Stellar Spirit mage, who wanted to join Fairy Tail. So they brought her home and then, after getting along unexpectedly well, the two decided to create a team with her. When the trio arrived at the guild, Gray introduced himself in his usual debonair way, by politely asking to borrow her underwear. Loke's repertoire was a bit more polished and charming… at least until he spotted the keys on her waist, at which point the disguised Leo literally ran away screaming.
Of course, as time went on, the group came to know each other better and, as seems inevitable in these cases, feelings developed. Natsu and Gray saved Lucy's life several times each; Loke fought for her and she ended up saving his life (at some point in the future, he would be teased about this). All of them invaded her apartment on a regular basis, and hung out together in the guild just about daily. Even though Loke wasn't really supposed to visit unless she called him, he tended to open his Gate every other day or so to spend time with Lucy and his friends in the guild.
Natsu probably spent the most time with Lucy, but he was emotionally oblivious enough that he was the last to really figure out when his feelings towards Lucy began to change from friendship to something more. Loke got it right away, which wasn't such a surprise. Being a complete playboy for years had made him sort of wise about these matters, and anyway Lucy was totally his type even before she risked her life and defied the Spirit King to save him. The only reason he hadn't hit on her a long time ago was because he was terrified that she would realize he was a Spirit.
Gray wasn't quite so speedy to fall for Lucy as all that. It was more of a gradual thing for him, which began with him easily acknowledging her adorableness and ended with him being a big fan of it, to the point that other girls didn't really register anymore. He was never as popular with the ladies as Loke (as if that were possible) but he did have some experience, and wasn't one to languish in denial.
Natsu, however, spent quite a long time languishing, deluding himself that Lucy was only a friend to him. Well, not 'only' a friend, but not someone he liked romantically, at least. He'd only ever had a crush on Lisanna before, and her untimely death had sort of left him with issues moving on, though he didn't really broadcast this. Plus, if he'd ever thought about it consciously, he would've felt guilty because Lucy looked really similar to Lisanna in some ways, and she deserved better than to be liked just because of her resemblance to his dead childhood crush. Given all that, it was pretty easy to see that Lisanna's safe return from Edolas made things a lot simpler for Natsu. He still loved her, but only as a friend and now that she was back it was obvious how she was different from Lucy. Though Natsu's personality was just naturally the type that didn't understand romantic things, he figured out that he liked Lucy relatively quickly after that.
So there they were, all three of them. Having figured out that they liked Lucy, the next logical step was to try to see if she liked them too. Of course, Fairy Tail members weren't exactly renowned for their logic, and for a while none of the three made a move. In the end, it was only the threat of being overtaken by someone else that forced their hands.
There was this boy, see. He didn't really deserve to be called a man, because he was only a regular teenager and had never faced death of any kind. In fact, his simple civilian status had been the only reason he got so close to Lucy in the first place; none of Lucy's admirers had expected her to like someone not in the guild. But the boy worked in a café that Lucy frequented, and – perhaps because he had never faced any sort of death – he had no qualms about messing up his current relationship with Lucy in the pursuit of a more intimate one. He asked her out on a date. Lucy accepted.
The date went oddly. First the restaurant they'd planned to go to somehow caught fire and burnt down despite the firefighters being on the scene within two minutes. They decided to go swimming instead, given the warm weather, when a freak storm froze over that particular lake and nothing else. Ever innovative, the couple dashed home and got their ice skates. They were enjoying the out-of-season pastime quite nicely, when all of a sudden a ray of dazzling sunlight reflected off the ice and hit Lucy's date in the eyes. Not only did the light temporarily blind him, but it almost seemed like a solid force capable of making him trip and fall face-first onto the ice, breaking his nose. Lucy offered to accompany him to the ER, but the boy was getting rather spooked at this point, and pretty much ran away without even pretending that he was going to call her later.
Lucy, being actually rather intelligent, had easily figured out the perpetrators and summarily dealt with them the next morning. Really, for the weakest member of the 'strongest team of Fairy Tail', Lucy could take down most of the other members easily enough when she got angry. …Though that was probably helped by the fact that instead of trying to fight back, they cowered in fear and begged for mercy. In their defense, her angry face was scary.
After some screaming, some smacking that hurt none of them but they were all too smart to tell her that, and some blatant lies on their part regarding the reasons for their involvement and whether they would ever do something of the kind again, Lucy gave up on them and stomped away to complain to Levy and Mirajane.
For a moment, Natsu, Gray, and Loke merely slumped against one another in relief. Then, it dawned upon all three that all three had gotten involved – meaning that all three held a markedly nonplatonic interest in who Lucy dated. They whipped away from each other immediately, pointing accusing fingers and, in Natsu's case, making faces oddly similar to a fish.
Happy swooped down and remarked on this. Paw at his chin, he began to ponder whether or not it would be a good idea to go get some fish now. Natsu was making him hungry, but it would be lunchtime soon anyway and Mirajane had promised him a special treat. Hm… choices, choices.
Loke ended up breaking the silence first, because he was eons older and more mature and also very versed in the ways of romance. "Dibs!" he said with a quick grin. "She's all mine."
"H-hey!" Gray protested. "You can't just… dibs her!"
Had Loke been less dedicated to maintaining his cool persona at all times just in case some attractive girl might happen along to swoon into his manly arms, he would have wrinkled his nose and stuck out his tongue in response. As it was, he just grinned smugly. "Can and did."
"You can't dibs a person!"
"You can too–"
"Then she's mine."
At this interjection, Gray and Loke both spun around to face Natsu incredulously. They were shocked that he was even claiming Lucy was his. Neither would've expected the Salamander to be able to understand he was attracted to Lucy in the first place, let alone shamelessly act on such feelings. Especially in such a way as he proceeded to do now.
"If you can dibs a person, then I get Lucy," Natsu said with an air of casual superiority. "I dibsed her a long time ago."
There was a silence, in which Natsu smirked about, Loke gaped, and Gray slowly narrowed his eyes.
"I don't believe you," he finally said. "Since when have you liked Lucy, anyway?"
Natsu flinched visibly at his rival's bold use of the word 'like', but held his ground. "Since none of your business, droopy eyes! And anyway, I didn't need to like her to dibs her. I just dibsed her to be on my team."
There was another short pause. This time the silence was a little less astonished, and a little more scornful (though in Happy's case, it had devolved into a fish-and-ice-cream daydream).
"Well, if that sort of thing counts as a dibs," Loke sneered, "then I've got way more claim than you anyway. I" – he puffed his chest importantly – "am Lucy's Stellar Spirit. She's got my Key."
(He made this sound incredibly dirty.)
"That's way more of a dibs and more of a team any day," Loke finished. From the looks of his expression, it was all he could do to resist adding, 'so there'.
This blow hit Natsu hard. He sputtered, "W-well, she never would've gotten that if it weren't for me! I brought her to Fairy Tail!"
Natsu and Loke continued to squabble ("What, you think she wasn't capable of getting here on her own?" "Are you kidding me? She'd do something stupid and get kidnapped or something! She was already getting kidnapped when we met!" "Since when are you one to talk about doing stupid things?"), getting louder and louder. Gray watched in silent trepidation, suddenly terribly aware that unlike both his friends and, apparently, rivals, he had no claim on Lucy. No dibs at all! He couldn't start a relationship based off no prior claim when Loke and Natsu were right here, who did that, it would be like going out with some random stranger …like… a boy who… worked in a café she frequented…
"HA!" Gray shouted, interrupting Loke mid-sneer ("I'll have you know I make an amazing gardener should the situation call for it!"). "I have the best dibs of all of us!"
"…What?" Loke asked in disbelief.
"Yeah, you don't have anything," Natsu contributed scornfully. "Psh, you're just a droopy-eyed third wheel. …Fourth wheel? Fifth? How does that work again?"
Loke and Gray both took a moment to aim a deadpan stare his way, not really surprised anymore… but still, honestly. But Gray quickly shook it off and returned to his point. "No, seriously," he explained. "I've got the best dibs out of all of us… because I have no dibs. Lucy doesn't like guys with dibs! She went out with that coffee guy without even knowing his name!"
"Actually, she did know his name," Loke said. "She kept going on and on about how Kyle made the best lattes."
Completely ignoring this, Gray continued triumphantly. "He wasn't even in Fairy Tail! She definitely didn't have his Key or was on his team. She's gotta like guys without dibs – so obviously, I'm the best choice out of the three of us."
"Dibs," Gray then added, just to be on the safe side.
Natsu and Loke's reaction to this was horrified, indignant, and believing all at once. Loke in particular seemed to be doubting himself. "You can't dibs based on not having dibs! She… wouldn't? But…"
Natsu, on the other hand, was a bit surer of himself. "Who cares what Lucy thinks?" he demanded. "I have dibs, I get her on my team, and none of you get to have her!" Unlike Loke, Natsu was most definitely not too mature to add the "so there!" to the end of that claim.
"I still don't think you even like her," Gray scowled at his long-time rival. "You just don't want anyone else to have her. You can't share!"
"I CAN TOO!"
"Ha! Prove it!"
"There's no way to prove that!"
"You know, I share her every day with all the other Stellar Spirits. I even share with Taurus, Natsu. Trust me when I say, you can't share."
"Shut up, Loke!"
"Oh yeah? Well, I share her with Erza and Happy and even you, Natsu! You didn't even want to do that!"
"I just didn't want to be on a team with you!"
"I didn't want to be on your team either, jackass!"
"Obviously, I share the most. I even used to share myself with twenty different girls, so I'm the best sharer of all."
"SHUT UP LOKE!"
"Why doesn't Lucy do that?"
The argument (which was getting dangerously close to a brawl) stopped at the new voice, and Loke, Gray, and Natsu turned in consternation to Happy, who'd been sitting there the whole time. Their shouting had finally broken him out of his daydream, and he'd come up with a brilliant plan after hearing Loke's latest statement. This way, he'd be able to see all of his meant-to-be Lucy/Loke dreams come true (!), but Natsu wouldn't get upset about it! And Gray, too!
"I mean," he said, twirling his tail diabolically, "why doesn't Lucy share herself with all of you guys? That way, everyone gets dibs."
Obviously, this suggestion was absolutely ridiculous. Not to condescend, but Happy was a cat. He didn't know anything about romance. Lucy was actually a sane human being, in a world where monogamy was highly valued. Sure, she was also a member of the guild that could probably have doubled as a mental asylum – but there was no way she'd ever agree to such an idea, and anyone suggesting it in the first place just deserved to be laughed at.
But this is the point where the story gets distinctly less typical. Because Loke wasn't even human, really. Natsu had been raised by a dragon. And even Gray's guardian had transformed him into a compulsive stripper. None of them were even remotely normal, nor did they have any idea of what normal consisted of.
To them, Happy's idea made perfect sense.
"…You're a genius, Happy!" Natsu exclaimed, and hugged his cat tightly.
Loke instantly took advantage of the situation. "We can each have her two days a week – I want Friday and Saturday!" He then snickered and mumbled to himself about 'the night things got crazy'. Natsu and Gray ignored him.
"I'll take Monday and Tuesday, then! I go first!" Natsu exclaimed, grinning widely. The homicidal aura he'd been emanating towards his fellows only moments before was totally gone now.
Gray, however, was neither (very) perverted, or (much of) an idiot. "Wednesday and Thursday are fine for me, I guess…" he mumbled. "But… will Lucy really agree to this?"
Natsu and Loke stared at him blankly. The idea of Lucy having any input whatsoever in this issue had clearly never crossed their minds.
"…Um. Of course she will?" Natsu tried, hesitantly.
"Well, I can't imagine her saying no," Loke scoffed. "I was always happy to share myself before."
Gray didn't dignify that with an answer. "And anyway, what about Sunday? That plan still leaves Sundays when she can… go have lattes with Kyle, or whatever. Do we really want to share her with a guy like that, too?"
"That's easy," Happy said, somewhat eager to finish all the negotiations and just go eat some fish already. He was seriously hungry by now, but Natsu wasn't letting go of him. "I'll be her boyfriend on Sunday."
They all stared at him. Happy puffed himself up, starting to like the idea more and more. "Yeah, I'll do it! We can eat fish together and… and I bet Charle is going to get so jealous!" He giggled and began to squirm with pleasure at the thought.
"…Okay, Happy. You can have Sunday," Loke finally said, after a long pause. He had an odd expression, almost as though he was trying not to laugh (which was just silly seeing as he was the Lion spirit and so really just a big cat himself).
But Natsu and Gray seemed to go for the idea as well, since Happy was better than a café boy any day, and the conference finally ended with all four of Lucy's new boyfriends spitting on their palms (and paw) and exchanging a solemn shake to agree on their new confederacy.
Lucy, meanwhile, was laughing into her soda with Levy on the other side of the room. She hadn't been too incredibly into Kyle, and by now the events of the day were more amusing to her than anything else. Besides, she thought, did she really even need a man right now? She was doing just fine as a strong independent woman, thank you very much, and boyfriends were over-rated in the first place. All she needed was her friends, thought Lucy, her female friends like Levy and Mirajane and Erza, but also her male friends, even if they were ridiculously overprotective. They were more important to her than anything else, and if having them around meant she couldn't get a boyfriend for a while… well, she'd pick them first any day.
Glancing over at the group of boys on the other side of the room, she smiled at them affectionately. They seemed to be getting along well, probably congratulating themselves on saving her from someone they deemed less than worthy. Really, they were so sweet. They just had her best interests in mind, even if they were all idiots about expressing it.
…In the huddle of boyfriends, Loke was negotiating just how far he could go with Lucy: Gray was claiming that kissing was it until he'd had his chance too, while Natsu said that they should all just wait a week and go on missions first instead. Happy was trying to retroactively demand a daily fish as payment for agreeing to take on Sunday boyfriend duty.
Poor Lucy Heartfilia. She had absolutely no idea what she was in for.
Gray was on his way home, when it started to rain and Jjuvia popped out from behind a building. Oddly enough, despite the rain that was usually an indicator of her feeling sad, she had a big grin on her face.
…Actually, it was a bit creepy. Gray edged back.
"Jjuvia heard about Gray-sama's agreement," she said, stepping closer and closer. Her grin began to falter as she said, "H-how could Gray-sama be L-Lucy's boyfriend?"
He gave her an awkward smile. Jjuvia really did look sad. It would be more moving if he had any idea how she knew about the dating agreement when Lucy herself hadn't been informed yet. "…Yeah. Well, you know…" he mumbled awkwardly.
"But why?" She demanded. "Jjuvia loves Gray-sama much more!"
Gray kind of made apologetic noises and tried to sneak around her. They were only two buildings away from his apartment. Maybe he should just make a break for it.
"T-that's why…" Jjuvia continued, blushing, "Jjuvia should get to share, too!"
Gray blinked at her, honestly dumbfounded. "You want to share Lucy? But there's only Sunday left. I don't think Happy's going to give it up. You could ask him, I guess…"
Jjuvia shook her head furiously. The rain fell a little harder. "Jjuvia wants to – to share Gray-sama!" she blurted, blushing furiously and yet smiling very excitedly. "Jjuvia wants Friday through Tuesday," she said.
Gray stared at her. "Oh."
She moved closer. "Gray-sama won't say no to sharing, right?" she whispered, eyes wide and hopeful. "Then Jjuvia won't have to go murder her friend Lucy!"
"Uh – I…guess not?" Gray managed, a little terrified.
Jjuvia cheered, the clouds instantly dissipating, and latched herself onto his arm. Gray flinched violently away, but her grip was unyielding.
"It's Sunday," she said directly into his ear, her breath tickling him and making him jump. "Gray-sama is Jjuvia's boyfriend today. Jjuvia will take him out on" – she squealed a little – "their first date!"
Then she began to drag him away. Gray found himself unable to resist, too shell-shocked and honestly frightened to tell if he was horrified or kind of pleased at this new development.