Feel free to jump to the TLDR

So a while back my GF and I were discussing kinks and stuff, which brought up her fantasy of having a threesome with another woman. Later, I was hanging out with a friend of ours who is a lesbian, and mentioned this to her so I could ask her for advice on the subject. couple hours later, and she bluntly tells me, in no uncertain terms, that she wanted to join us for the FFM threesome my GF wants. I should be all over it, excited by the thought, but unlike most guys in my situation I'm not over the moon about it, yet I don't dislike the idea at all. I'm a bit aroused at the thought of it and I'm certain I'd enjoy the act too, and we totally trust the other. Just the same with myself, no matter how pleasurable it felt, she'd remain loyal to me after the act. We also made it a point communicate openly on any problems immediately- discussing it's every aspect, specifics, exceptions, and any of our personal caveats involved. Additionally, while she is a close friend of ours, if either my GF or myself felt after doing that this friend was in any way attached, clingy, or otherwise a danger to our relationship, we'd cut her out of our lives entirely. In short, it wouldn't damage our relationship, it's not a negative for me, and cannot think of any good reason to not go ahead with this.

Knowing all of that, I decided I'd honestly support my Gf on this one, so how can I move forward with this situation?

TLDR: gf would like a threesome with another woman, told my lesbian friend about this, she volunteered to join it. Need advice, please and thank you