Interesting, so I was able to get the FFM threesome on the Throuple path with slutty Holly and slutty Lena. I'm unsure is this is a feature or a bug. I'm leaning towards the latter however. How I did this was by sleeping around with both protagonists and not getting into a dedicated relationship. In chapter 12 Jeremy tells Ian of Lena sleeping around (with him as well). Ian chooses that he cannot see Lena as just a friend. Lena decides she still wants to be with Ian. Normally this would be the trigger to enter into a relationship with Lena late but...Holly confesses to kissing Lena so on the beach, after the never have I ever game, instead of the relationship talk with Ian and Lena, I get the threesome scene. Bimbo Alison messages Ian next morning and Ian doesn't say he is in a throuple relationship now like he does on if he had the relationship talk with Lena last night. Also Ian says he's uncomfortable with his relationship with Lena after discovering her deceit. I thought this was fixed? Next chapter, for some reason Lena can have sex with Stan. The chapter 13 threesome scene happens. Then next night Lena asks Ian for a threesome with Jeremy. Then the "glory Holly"
scene happens. So like I said, these things are likely bugs but just throwing it out there.