Hey whyler,
This isn’t all that difficult, so many women have these fantasies.
For starters, you will need to target the right venues, where like minded ladies may hang out. Next, I suggest one of you approach a target, or if you’re together, casually make a passing comment over your shoulder, then if the signs are right continue to engage. If you’ve got the charm (chose your pheromones carefully
) I suggest you open a set on your own, when your girlfriend is not present jokingly complain about how horny your girlfriend is for another woman, as she used to be a lesbian etc. Then ask the target for ‘suggestions’ to remedy this. She will either indicate an interest in meeting your girl, or she will not. After a couple minutes your girlfriend should arrive and say hi, just for maybe 15 seconds. Do a quick intro, to build some initial comfort, then your girl should point to another side of the room, like she is involved in an interaction over there, and leave. Joke further with the target that your girl is probably picking up. In a few minutes your girl should return and its a good time to isolate the target, buy her a drink etc and get into further conversation. At some point, you or your girlfriend should whisper in her ear that there is an interest, or use bodylanguage to communicate a romantic interest. Now, you will need to find a reason to bounce, preferably back to your home – hopefully it’s not far away, this will make it easier. The reason can be something simple like showing a dvd collection, photos, more partying etc. Once at home, to get things moving, you come up with the ‘eureka’ idea to do “dual-massages” on each other, which can feel fantastic with 2 sets of hands working a body. Give your target plenty of attention. When you and your girlfriend massage her, you and your girlfriend should kiss. Then when the target and your girlfriend massage, that’s when your girl has to pull the target in for a kiss. Then, you’re on your way to an ‘innocent and spontaneous’ threesome, hopefully 
This strategy can be shuffled around in so many ways, ie your gf does most of the communication and builds most of the comfort. Kiss games can also be good at a club for instance, with the goal of getting your girlfriend and another girl to practice kissing on each other in order to ‘test’ how good of a kisser they are, etc.
These are just a couple techniques which can work. I hope this helps, good luck 