threesome?

Hey guys, so me and my boyfriend of 2 years today just thought about maybe having a threesome. But we have a few things in mind. 
One- We both want it to be M/F/F. We think it would be easier Considering I would get to have my lesbian experience and we would just be comfortable about it. PROBLEM WITH IT.. I'm worried he might end up falling for her. And yeah I've told him and he said no and said he will always want me more. It's just that fear. But we both wouldn't mind trying it. And also, it's hard finding someone who he wouldn't take the girls virginity and just finding someone we both trust enough not to tell or even be whiling to do something like this.. so I guess I'm here to ask if any of you have any ideas on what we could do. And if you have had a threesome what you guys did to find someone or even just how you felt about it and stuff. Please comment