Threesome with my boyfriend and best friend.

So, last night my best friend and I were talking about threesomes and she mentioned if my boyfriend ever talked about one and I said the only person I would be comfortable with would be her.. and so I went on and told my boyfriend about it and his reaction was completely different to what I expected. He said as long as I felt okay with it he would be okay with it too. Now, he's coming to see me soon and I'm excited/scared at the same time because I've never done anything like this... should I go for it and hope for the best or just back out? Lol