Bf slept with his friend twice before

I'm 22 (m) and have been dating my boyfriend (22,m) since October.At the start of our relationship, my bf told me he has had a threesome before. I was accepting of that and we moved on...however a month after that he elaborated that it was with his best friend. He told this to me a couple days before a Christmas party where his friend would be.According to my bf, he's slept with his best friend twice. When I first asked him about it, he said his friend had boozed him up and gotten handsy. He didn't like it.This raised a red flag for me because at a later date he went and had a threesome with his friend.Recently we had a conversation about alcohol etc, and he again elaborated that the first incident occurred when he and his friend were very drunk. His friend had made advances and when he realized what was happening, he freaked and they didn't talk for 3 months.After that period, they ended up having a threesome. They were all drunk and according to my bf, he (my bf) was drunk and wanted sex. This makes me feel uncomfortable...the fact that he freaked at the first incident and then went willingly into the second. It raises a red flag for me. I told him that I don't trust him when he's drunk based off that information.Now, I've been seeing a therapist for the past month and I'm torn on whether or not I can trust my bf at all. Most everything else between us is fine...except I don't feel trust for him.I think that sleeping with a friend really changes the relationship in that friendship.I feel like my mind is going everywhere. I don't know if and how I can rebuild this sort of trust. How does one do this? Any thoughts and opinions would be appreciated. Thanks for reading.