Wow... I'm not quite sure where to begin. First, you guys really need to examine what you each really want and then COMMUNICATE with each other openly and honestly..A threesome is outside monogamy technically, but isn't yet Poly either. But without clear goals to start with, you will have a hard time getting anywhere.
The second big one I see for you is this prior FWB. Again, depends on what you guys establish as what you want and expect. You have ruled her out as part of a triad, but if he opens up and says he wants to develop more with her, are you ok with him pursuing her as a V? If so, is he ok with you pursuing another man as part of your own V, if you so choose?
Again, lots to talk about first!!!