Poly relationships can absolutely work, sex does not have to be about real love and commitment and regardless, there can be love and commitment between 3 people if everyone respects the other. However, it doesn't sound like you are being respected, and your established rules are being broken. You will have to consider what is best for you personally.

There are well-meaning posters who will say that a relationship can only work between 2 people, these people have been taught certain things and have certain beliefs. Take what they say with a grain of salt.