Is it normal that my best friend wants to be with me?

So my boyfriend and I have been together for 5 years and rent a house together. Now my best friend and I have known each other for about two years, neither of us have close friends just the two of us. But she had become flirty with me she would slap my ass and ocasionaly grab my boobs, I didn't know how to react so I laughed it off. But my boyfriend caught sight and he deff liked it. She did this more often, and he would try to bring her up during sex, he would get me to say her name while moaning and what I would do to her. I did this for him. I noticed they started texting a lot. She started coming over everyday sometimes stay over, we became close sharing drinks etc. My bf actually started paying for her when we go out. Like movies and dinner the three of us. I brought up to my bf I don't know how to handle the grabbing and flirting she does to me, he responded kiss her. He kept pressuring me to kiss her for a couple of days. One day I did it and we made out for hours I loved it. But I thought that was that, but my bf wanted more. She started kissing me on the regular, I don't know how to stop it. But yesterday she told me she wants to be with us and that she is in love with us. She wants a three way relationship and move in(She practically lives here now). I don't know what to say, I brought it up to my bf, he loves the idea. She left this morning and gave me a kiss goodbye and then one to my bf. I didn't say anything, I don't want to lose them what should I do?