I think I have both branches active.Just use the Jill path, the Bella's breakup is simple and (for now) without consequences... Think in this:
If I have to choose only one between Bella and Jill, Bella is the more seductive one to me.
(Bella is experienced, Jill is so naive)
Besides, I guess that if the threesome is even possible, it's much more likely that Jill can be persuaded by Bella to share Bella's boyfriend than that Jill can be persuaded by anyone to share her boyfriend with Bella.
It would be totally out of Jill's character to go straight from shy virgin to polyamrous.
Instead, if the MC rejected Jill chosing the "because I can't have you both" option, maybe once Jill knows that Bella is OK with sharing the MC with her, maybe it's easier for Jill to accept it.
I think that this would be out of Jill's character too, but less than the other way around.