You say she is cheating. Those who cheat with you will also cheat on you.
Also, the relationship you have now is not polyamorous, since poly is rooted in honesty, integrity, and respect for everyone involved. Neither she nor you are demonstrating much respect and concern for her OSO. At the moment you both are treating him like a mushroom, in that you are keeping him in the dark and feeding him a load of shit.
If you want to move your V forward into a state of integrity, insist on meeting him and putting it all out there. If she's okay with that, fine; you all move forward. If she refuses, there's your answer. If she says okay but the meet and greet never actually happens, beat feet, BrotherMan, just as fast as your feet will beat.