Yes, but the game pretty specifically makes it mutually exclusive for Lena to have sex with Jeremy and be in relationship with Ian. I don't know what's Eva's reason for this, but making an exception for this for a threesome when prohibiting it otherwise would feel rather counter-intuitive to me.But Lena could still have given him a blowjob during the drinking game, watched him fuck Louise, masturbated thinking about him, and exchanged nudes with him.
I mean, with Holly, which has somewhat similar arrangement with route blocking, we have Lena repeatedly acknowledge this and narrate her reasoning. And we have
lena_go_holly
variable to track Lena's interest in Holly, even if she holds herself back. But with Jeremy there's no such thing, if Lena enters relationship with Ian then she basically acts as if nothing she did with Jeremy ever happened, there is no further interactions and she doesn't spare him any thoughts. So going from this to "hey, how about a threesome" would be a bit weird.