If you are open to a bit of gray area you could cross it off with an asterisk.
FFM: you, her plus other her w/strap on.
Did this a few years back with a lesbian couple I had met. Truth be told it was just meh for me. Couldn't really get good rhythm, felt crowded (legs) and generally just awkward. The novelty was short lived (for me anyway); you don't get the same view as you do in porn. It simply felt like I was just holding a place in line with an occasional bump. The girls seemed to really enjoy...so there that I suppose

The caveat is this was a first for all of us and I stayed upfront. Maybe I missed some hidden sensation but overall have not been in a hurry to try again.