I don't know what to do

Me and my fiancé had broken up and I had left for a few weeks, in that time I had sexual relations with somebody. I can't say that I don't regret what I did because the person is truly a great person. Our break up happened due to a lack of communication, intimacy, sex for over a year. My fiancé begged me to come back home saying that he's changed, he would listen to me, communicate with me better, not be verbally abusive, clean up after himself better, etc. So I came home, I told him everything I had done with this other person and he said he'll forgive me if I have a 3some with him and another girl, but he wants me to pick a girl.. I really don't know what to do. I definitely don't feel comfortable asking anyone to do what he wants, if anything I would rather him just do his own thing with another girl and me not be around. I love my fiancé and I always will, but I am not in love with him and I haven't been for a long time, which I have already told him.