I think you shuld discuss it with your BF. You need to find out what you need. I would also talk to your friend and see how she feels about you. If you have had these thoughts for a long time you need to deal with them. I think it is a beautiful thing for two women to become involved and share a mutal passion. I would not recommend a threesome just yet. You need time to find out how much you feel for your girlfriend. Is it just sexual or do you have an attachment to her that goes beond sex? If you find out that you are in love with her and your BF, you need to discuss where to go from there with everyone.