3some

Boyfriend and are just Talking about it. I feel like I wanna do it but at the same time thinking about him fucking another girl makes me cry.... Like how would he feel of we had a 3some with another guy... Instead of a girl... Anyways my real question here is...
-how do we go about this?
-if you have done it is it with someone you know like a best friend?
-or how do you find someone?
-were condoms used?
-did you back out right when it was about to happen?
-like I'd so do things with another girl if he wanted me to, but his penis in another female makes me want to cry.... 
-idk I need help:/