Look you gotta think about it this way
A: even the mmf wasnt something you guys did so far at one point. And yet you tried it, and obviously liked it since it kept going
B: this is a great showcase of faithfulness. The fact you know there is no chance this is her wanting to leave you, and you know its just a sexy kink you guys got, shows that you 2 have a very good and trusting relationship
C: you are not sure how much you would like it. Were you 100% sure you would like the MMF when you first tried it?? if not, why is this any diffirent. Let her try it once, if you dislike it, be adult enough to just talk to her and say "honey, we tried this, i dont like it, no more of this".
about "too much" commitment, im not sure what you mean by that. You dont really gotta commit to anything. The thing about fuck buddies is that they are not tied to either of you in any way. At any point you can tell that third guy "ok enough, my wifes legs are closed, go away" and he is out of your marriage