Has she expressed why she doesn't like you? Is guy Z taking issue with you only because girl X doesn't like you?
What does your partner propose to do to try to manage this concerning the feelings of all involved? I mean, I get that he is a top and wants someone submissive to play with, but why HER if she can't accept you? she isn't the only submissive girl on Earth. Did she ever like you?
I guess I'm looking at this -
She was with your primary before you came along but she had guy Z with her even then. This says she is familiar to some kind of open relationship/poly enough that she expects both her male lovers to accept that.
But then one of her male lovers begins a relationship with another (you) and she isn't applying the acceptance to it she expects out of her partners?
That just leaves me to wonder what her grounds for not liking you are.....