concept: jason is the first person both piper and nico ever have sex with, and the first man leo sleeps with. piper and jason happen while they're in a relationship. nico and jason happen shortly after jiper break up when nico asks jason to sleep with him because he trusts him and wants to be prepared once he and will get together, maybe after a monthly argo ii meetup. it's a trust and comfort thing, happens a couple of times at most. once leo comes back, things are complicated with calypso and they're both emotional over seeing each other alive again that things just kind of happen, just the once.
then once jason dies piper and leo and nico all kind of fall together, depending on each other and mourning together. leo and piper were already best friends, and nico and leo had some bonding over how isolated they felt on the argo ii, in addition to being queer and the trauma over losing their mothers and just trauma in general tbh. then piper comes out, and nico ends up bonding with her, after holding a bit of a petty grudge over her dumping jason.
nico and will's relationship falls apart in nico's grief, so does piper and shel's with piper mourning and generally feeling conflicted. leo and calypso don't work out for a number of reasons (because really, that relationship was doomed from the start imo, be so real rick).
somewhere along the line, the three of them end up falling into a queer-platonic relationship, that eventually stops being platonic as their casual intimacy escalates: cuddling that turns into sharing beds that turns into friendly pecks that turns into kissing. and leo finally takes the step and admits he's attracted to both of them, and he and piper are both bi but nico is gay, but he's fine with kissing and touching piper, and piper kissing and touching him, because it's fun and they love and trust each other and yeah. leo/nico/piper poly.