Ask your girl if she's ever thought about being with another chick. If there's even a slight hesitation in answer, you're good to go.
Step3
Start mentioning it to your girl about once a week.
Step4
Get her to do a little threesome dirty talk when you have sex.
Step5
Ask her after a couple of drinks which one of her friends she would hook up with if she had to and talk about it for a while. During the conversation mention that a threesome is one of your fantasies and ask her if she would talk about herself doing her friend while you have sex (she's already used to that kinda talk, just get her to make it a little more real).
Step6
Again, have a couple of drinks and bring it up again. But this time ask her if she would want to have a threesome with her friend. If she says no, keep working on step three and she'll eventually come around.