Welcome to our forum.
I'm in agreement with GalaGirl: You seem to be better suited for a V relationship than for a triad relationship. Whereas your husband and his girlfriend are pushing for a triad. You should probably sit down with them and very carefully explain that a triad is not what you want. Perhaps a compromise would be what's called an emotional triad: still technically a V, but the two "arm persons" have a very close friendship with each other.
It sounds like trying to talk to them can be difficult. I feel for you in that case; communication is so important in a poly relationship. I think you still have to keep trying at it, perhaps getting more practice and getting better at it. I don't know anyone (myself included) who couldn't stand for some improvement in the communication area.
I would just tell them that I know this is hard, but it still needs to be talked about. Hang in there.
Love means never having to say, "Put down that meat cleaver!"