I think that would have the potential to get messy. The only 3somes I've had were with all girls. It was SO great. We were all best friends. We'd get manicures done, borrow each others' clothes, and fuck around. I'm not sure if anything is much more heavenly than being with 2 women. It's funny but none of us got jealous about the others hooking up with guys-- but if another girl came into the picture, we would get pretty catty.
I couldn't handle having a 3some with my bf and someone else. I know myself well enough to know that I'd flip out if someone touched him. I've always told him that I didn't care if he messed around with guys but that mostly pisses him off because he wouldn't do that (I think that's kind of closed minded but it's his call). If I was single I would be up for a male/female 3some but only if the other couple I weren't close. It would have to be me as person #3 because I couldn't handle sharing my bf. I suppose it's not an issue though as I'm single and don't cheat.
If you think that there's any chance that things will get messy emotionally and you'll end up hurt or somehow damaging a relationship that means something to you, it's not worth it IMO. Friendships and romantic relationships hold infinitely more value to me than any sexual adventure ever will....though those adventures can be damn fun....