since i was unable to find the post that i was thinking of, here it is by me instead:
trios where it's the couple + one of their siblings. and the sibling is there because their sibling wants them there. and and because the relationship-sibling's significant other wants them there.
this is literally Hyapollo + Polyboea
Polyboea gets invited on outtings and she and Hyacinthus race each other because Apollo leaves them in the dust every time and she and Apollo laugh at Hyacinthus when he snaps a string on a lyre and they have hang-outs like any other young adults/older teens would do and exchange gossip
they WOULD have movie dates. polyboea gets her own popcorn bucket (it is a JUMBO) and munches on it while Hyapollo do the couple-sharing.
random person who can't see past their nose: what a pretty lady!
polyboea: thank you <3
random person who can't see past their nose: and which of you fine gentlemen are the third wheel?
polyboea: that'll be the pretty lady 😉