I think that Zara will not support the throuple as she is just Too competitive - so she is a 'pick one of us' for the relatinoship.
I Could see Nadia being brought into a 3 sum by both Bella and Zara.
The questions is whether or not Nadia would accept being the more submissive of the two women when with the MC.
Would she be too awkward to take the lead or would she just concede it to either Bella or Zara in the interest of trying out something new with women (and the MC) who she felt safe with?
I think the MC would Not allow Nadia's banana anywhere near his backside... that is no go territory!
That same banana could easily become more intimate with Nadia, but would she still want to eat half with the MC afterwards...