Only threesome that should be with Bella should be Jill. He's hinted at a past thing between the two if you dump Jill for Bella, you still get a kiss from Jill If you say, "Because I couldn't have both." Bella was "uncomfortable" sharing a bed with Jill because of something Jill did in the past that she "Wouldn't do again." It's been so long but I believe Bella says this can't happen again referring to a relationship that was untoward at some point I might play again to get screen grabs of these things but I'm 99% positive of everything but Bella saying the word "Again" in reference to a "Student relationship"/"Or some other kind of inappropriate relationship" I'm banking on her husband leaving her after finding Bella and Jill kissing in bed at some point NOTHING TO MAJOR of course just wrong moment wrong time blown out a proportion.
Also as to make it make sense. [Bella loves Jill every bit as much as she loves MC so Seeing Jill heartbroken isn't something she'd enjoy for anything.]