Although I'm very late in saying it ... welcome to our forum. It sounds like your poly experience so far has been a pleasant one, and I'm glad to hear that.
This guy's girlfriend, do you just have a platonic relationship with her? If so, you're in what we usually call a "V" relationship ... the same configuration I'm in. If all three of you are romantically involved with each other, then it's a delta, triangle, or triad. (I know I know, too many labels
Hope you enjoy your stay with us.