Honestly, it's a wonderful fantasy...but if you really think no cumming and no kissing is gonna have ANY impact on any possible jealousy factor, you've got the wrong end of this. The point of a threesom is to have fun. If the rules boil down to "don't have toooo much fun," then I think it's better that you get the full fantasy potential out of it and re-evaluate after a while. It's doesn't have to be the relationship ending experience you're worried about, but it can strain a relationship if folks aren't ready or right for it.

As far as regrets go, well....you can't do everything you've ever thought you might want to. And missing out on a lifelong love is, to my mind, a much bigger loss than a threesom you weren't all that sure about.

As a random thought, you might try chatting together with a girl online....provided you don't consider cybering cheating....see if that changes your mind about it one way or the other.