MAKING A LONG STORY SHORT

It all started when my boyfriend suggested that I ask my BFF to join us, I was very hesitant at first but I decide to ask her. I sent her a text; she wasn’t into it at first but agreed because it’s something she wanted to experiment and she would do it because I asked her to. The first time was a little weird but we figured with time it would get easier. We talk and text for months between the 3 of us I was not aware that they where having conversations and lying to me about it. When I found out I confront them and told them I was not ok with it and want nothing to do with them anymore.

Months later my boyfriend convinced me again to start talking with her and make her my girlfriend we talked about it and she agreed weeks later he mentioned that he felt left out and want her to be his girlfriend as well she agreed and I didn’t under the conditions that they would not talk behind my back or lie to me. Weeks ago I found out that they are in fact talking behind my back AGAIN saying I love you and calling each other sweet words they don’t say around me. She is now living with us and I don’t know what to do. Before she moved in I found out they where on the phone together I texted her and ask who she was talking with and she said one of her aunts asking about some money I say ok no problem (she was lying to me) I ask my boyfriend if he have been talking to her he denied it. I can see how her face light up when he’s around I’m paranoid that someday they might end up sleeping together if they haven’t by now and I don’t know what to do. Can I trust her? I know I can’t trust my boyfriend I apologize for my English it’s not my first language and their is way more to the story of them being sneaky.