My first threesome was with an bisexual couple, both women. They weren't really lesbian, although they would sleep together occasionally, and each would hook up with men, too.
I got to know one of them pretty well, we had hooked up a couple of times through work. We worked different places, but got together after work a few times. We liked each other's personalities, and at the time I was a thin, strong pro athlete and sought after by lots of women (not bragging, but aware of it by reflection some now 30 years later). She was great sexually, and I learned a few things from her - I was still young and wild then, as you might guess.
After a party one night, she asked me to stay, which was not unusual. But when we locked the doors and moved into her bedroom, her friend was already there waiting for the both of us. I learned a lot that night about pleasing women, lessons that I took to heart and continue to apply to this day. We shared ourselves sexually several more times, but soon we all drifted away from each other and that was that.
I would love to find another couple of women who would like to teach me a few new things. Sex with two women focused on each other and finding ways to include me (that is, I was not the center of attention) was extraordinarily freeing. What I would give to experience that again.