I commented it as just an example, but I think I might be onto something. Likes girls is necessary for lesbian, lesbian is one way to get into a threesome if you manage to sneak up and actually interact, there are also positions in threesomes that require the girls to have some lesbian action at the same time, like one riding the player's face while the other rides his dick. So maybe likes girls is required. Just checked the F5 help slides and it does mention that the ability to do threesomes is chance based, it will not appear every time even on the same girl, but that chance increases if she has already been in one before. So maybe you are just having bad luck with that chance mechanic as well. It also states mood, whether she's angry basically, and personality DO play a part in the ability to do threesomes.
I know. But they did have the Likes Girls trait. They were also maxed out affection and lust, and not angry.
It's possible RNG shafted me, but despite the F5 menu saying it was only a chance, I've actually
always gotten the threesome option from fully experienced girls, and
never from girls who weren't fully experienced (even if they were 100% in everything
except one thing, because it took me far too long to figure out how to Nipple Tease instead of just Breast Tease).
In addition to the custom girls I made (who
did have Likes Girls, Slutty, etc), I was using a number of presets with a variety of traits and personalities. None of them would ever do a threesome until they were 100% experienced, despite being at max affection and lust, and not angry, and being my girlfriends, and being members of Koikatu Club. Then once they were experienced, all of them would do threesomes all the time.
This was the case for both threesomes initiated by finding two girls having lesbian sex, and for threesomes initiated from a normal H scene when you feel someone watching you and you can invite her to join in.