Threesome

So my boyfriend has been going on at me for a three way for a while now and to be honest, I’m kinda into it too and I think it might spice up our sex life a bit. But, he keeps saying we should ask one of our best friends. It’s her last week with us here before she goes back home. I understand doing it with a friend as we know her and it would be fun but I also don’t want to loose our friendship over it. I also wouldn’t know what to do if my bf and best friend started doing something and I wasn’t happy about it like what would I do when that started to happen? Also he’s asking me to ask her to do it tonight. Any advice would be much appreciated. Thanks.