The way I see it;
1) He cheated. How did you deal with that?
2) You 'allowed' things to drift into polyamory because? You were afraid that he would otherwise leave you? She was enticing? It was easier than to deal with the pain of him cheating on you?
3) She was not interested in a relationship with you. Do you still identify as monogamous by default or do you see some for of consensual non-monogamy working for you too?
4) She moved in 'just because'. Big red flag! She needs to get her own place. It's not fair to force you out of your apartment so that she and him can enjoy their NRE with no interruptions. Your post reads as things moving blindingly fast and you being left with wondering 'What just happened with my life?'.
So what to do now?
1) Deal with the cheating and the hurt it has caused.
2) Really discuss polyamory this time. Boundaries, foundations, time-management, separateness vs. togetherness, all that jazz.
3) Consider how you can have your own needs met, sexual and otherwise. Do you want to have casual partners for sex and affection on the side or explore a full-blown committed love relationship with a possible future secondary of your own?
4) Move her out. Actually, this needs to be done first.
Sorry to sound a bit curt, I just lost a giant post and tried to recap from memory