itch.io

At the party you can get someone approach you and ask if you want to join them and another character. I think there's also a post-football match encounter that can result in a threesome (at least as a cheerleader, I've not tried as a player). One of the ways you can resolve the Esports team story has a repeatable threesome encounter too.

There may be more, but that's a few to find for you.

get them into a Polyamorous relationship. Then ask them on date, Hang out as a Polycule. The date will end with sex with all of them simultaneously.