Wow, CFL. That's really hard to say. I like the idea of going with the flow but I also over analyze too. I just hope that two of you (you and your partner) are very secure because I imagine a lot of people like the fantasy but once they see their partner doing things with somebody else, major insecurities and jealousies could surface.
I just think you both should discuss what the ground rules are. Like what you will allow each other to do with the guest without getting upset. Also, I'd suggest smoking some marijuana or drinking a bit to diminish your inhibitions. I imagine hand and mouth stuff would be cool for both of you.
I've been talking about doing 3-somes with my partner for probably more than a year and a half. But whenever we try to find a woman we like, she's only interested in one of us. Usually me. Not both at the same time. It seems really hard to find a willing participant just to be a guest. My mission is just to find a woman to make love to my gf while I watch, or only touch my gf. I won't touch the guest. Anyway, that's the plan, but haven't found a willing guest yet.
We've discussed doing it with a girl and possibly a ladyboy (pre-op and post-op) but I don't know if I'm evolved enough to let a genuined manly man near my gf. I think I like to keep that in the fantasy zone. I think I might be able to deal with a very feminine pre-op ladyboy though. I'm sure we could find that possibility in Thailand

I've done quite a lot of 3-somes with 2 girls but both were strangers to me, it can be a lot of fun. But because they were both strangers there was no possibility of jealousy.
I'm curious and maybe you can help me. But how will you find a willing guest for a 3-some?