DON'T TRUST A FEMALE THAT HAS A FRIEND WHO TALKS ABOUT A THREESOME. Obviously your girl likes pussy. When all is said and done, she likes pussy more than she likes fucking you. I am just being real. If that is her close friend confiding in her about wanting to fuck another girl, you can be rest assured they got the hots for each other. I would just be wary next time they "go to the bathroom" together. Because now that the 3-some talk is out in the open, it makes you wonder what they were doing before they brought it to your attention, i.e...watching each other masturbate, touching each other, stuff they wouldn't dare tell you but since one is your girlfriend now- they want to bring you in on it. That's just me, because I fucking hate "bi-sexuals" and if my girl ever fucked around on me with another girl.........well let's just say they might not be around to tell the tale.
Once you do it, it will be open-season in your girl's mind. Because somewhere down the road you will be out of town, and being bored, she is gonna a have drunken night out with the girls. From there, especially with a little alcohol, you're girl is gonna be eating her friends pussy. NEVER TRUST A BISEXUAL.