The best places to find another girl for a threesome is the strip clubs or swinging clubs. Or you can do it the easy way instead by renting out an escort for a couple of hours.
I have had 2 threesomes in my life, but it wasn't with any g/f. I rather prefer one on one instead. I personally would never have a threesome with serious g/f because I have a feeling that it can have high potential of ruining the relationship. I have seen it happen before, but that's just me.
No problems here watching my girl get nasty with another chick, but nooooo way in hell am I going to watch some other guy stuff his hog into her. I don't need to bang the other girl, I just like to watch and help my girl get off anyhow, so it works out fine.
one night me, my girlfriend, and her friend were getting wasted and we just ended up having a threesome. Everything was good until I started fucking her friend. She got really jealous and left the room. Be careful how you play it out so you don't hurt your girl.