Hoveabode you're a lucky guy. I once asked my wife if she would be willling to have a threesome with me, and she said HELL NO! And if she every caught me fucking around, it's all over between us.[ I wish she would catch me, but than again maybe not, I don't want to become one of those mongers who go everyday to Tijuana, like a addicted sick dog.]
Sampson is right, be very specific. I have had many threesomes in TJ with bargirls, and a few streetgirls, it's a lot of fun, but for the most part it's fucking one girl at a time, while the other watches, or I eat out one girl and fuck the other. But to get the girls to eat each other out, and interact is VERY HARD to do. I'm sure your girl friend wants to engage in oral sex in both the giving and recieving END.
So you have to be very speciifc with what you want in AB, CC, or HK all three would be your best bet for that type of service.