ElipseWater Doctor Oddessey au???
Hear me out, cruise ship doctors Regulus and James who have been working together for a while. James is in love with Regulus but thinks Regulus only sees him as his best friends brother.
They meet the ships new head doctor one Remus Lupin and Regulus is immediately attracted to him and they hook up.
James is super jealous and thinks he hates Remus guts (but is just really attracted to him). He and Remus essential compete for Regulus attention (who likes them both).
Then the gay week episode happens and the three have a threesome. Regulus wants them all to be together but Remus doesn't know if he's up for a relationship like that and James decides he doesn't wanna have to "share" Regulus with anyone.
So James and Remus start trying so much harder to win Reg over to one of them with eventually leads to them arguing and hate fucking.
James and Remus start freaking out after that cause they can't be in love with TWO people! Can they?
And after so much pining and "casually" fucking the three idiots finally decide to get together and everyone lives happily ever after π
(Not to mention as well how easily you could make this an age gap situation...)