There are a few opportunities for threesomes in the game right now (these are just the ones I know about) :
If you're poly and schedule a polycule hangout it can end in a threesome/orgy.
If you keep flashing the construction workers in town they can invite you to have a threesome.
There's a repeatable threesome if you're part of the swim team.
There's a threesome with the 360 No-Scoper and the E-Girl in the esports storyline.
You can potentially have a threesome with 2 football players after a game if you're a cheerleader.
And finally there's a potential threesome scene when entering your dorm hall if you start with the slut inclination.
I'm about 3 weeks into my current save and I occasionally run into them, getting catcalled by them is a random event that can happen when moving around town. Just know that to get the threesome scene with them you'll need at least 5 disinhibition and to encounter them several times! Walking back and forth between the Riverside Plaza and Yohimbe Street (where the River Rat) is how I usually find them.
FFM is certainly a thing possible in the swim practice scene, or if you're polyamorous and invite a couple female partners for a polycule hangout. For the other scenes I know the genders of the characters can be influenced by the player. Like at the very start you can choose to make the gender distribution of your world all/mostly female, and make your character a hetero guy so characters with predetermined genders are more likely to be female. Though that stuff is kind of "in theory" for me, as I usually play a bi female with even gender distribution :)