I crave mmf

My gf (23f) and I (32m) have been together almost 3 years and in that time we've playfully discussed adding a third and while I am somewhat hesitant it turns me on to no end to think about getting another's cock wet and ready for her, getting roughly fucked up the ass, and cleaning up the mess after. The only real hangup is that my gf is only interested in the fantasy, she's too stimulated by the time I'm done with her and Idk if she could go the distance in an mmf. So that's where I'm at 🫤