My serious boyfriend wants to have a 3 some what should I do?

My SO wants to have a 3some with another girl I alway thought about it but never did it. He said he wants to do it before he's married because he always wanted to do it and wants to do it with me. Im not sure if it's a good idea my best friend said it would kill my relationship and it's not going to be good. What do you ladies think?