Bringing in a 3rd? good idea/bad idea

Hey I wanted some feedback from fellow Doms & even entertain a few opinions of subs...
My very good girl sub girlfriend and I were playing with the idea of bringing in a 3rd to please me by letting my sub dominate this little slut waitress we took a shine to. So the question is, while, what I say goes and she would do this willingly and happily; I am not wanting to fuck up a very happy taken/kept relationship. We both feel quite lucky to have found each other and while our play time is epic, our actual relationship is pretty damn epic too. That said, she is a woman with normal woman jealousies and other womanly emotions. I wish to know if anyone has any positive or negative adventures to share that shed light on the dynamics that can change if you let some mousey whore waitress suck the clit of your sub till she screams.

-E.d
Tough dom probs huh...?...

yah

I think we prob won't end up acting on it, but while she is 'straight' she really is considering the idea because of the attitude of the other gal. something about her just got her going. A lot of it is prob that this is her first sub relationship she's ever been party to and she's learning alllllll kinds of new stuff she never had the chance to try before. I am pretty proud of her as human being for getting to find out what she likes for herself. Just don't want that curiosity to end up causing the very things she hates about an unkept life. Petty jealousy and the like.
Thanks
-E.dave