Things to consider for a threesome

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So my girlfriend of 3 and a half years and I have raised the possibility of having a threesome with another friend of mine. My GF and I have talked about it before (she likes girls, never been with one though) and we both agreed that my friend would be a pretty good candidate for a third (really open with casual sex, done MFF and MMF threesomes before, etc). Last night we were talking with said friend and my GF brought up the idea of having a threesome. And my friend openly considered it.

Now I'm in a position in which this could actually happen and I don't want to fuck anything up. So what are some of the real concerns with having a threesome with my GF and another friend of mine? There's the obvious things, yeah (jealousy) but what are all the things to consider? I definitely want to talk all this shit through with my GF and friend before we pull the trigger and I'm getting my thoughts in order for talking points.