They don't call them unicorns by accident! Lol.This is why I think MFM threesomes are more common than MFF threesomes. More men are willing to do it.
A long term couple (either married or unmarried) could show up to a bar and find a single dude who is down to have sex with them that night. It would be much more difficult for that couple to find a third female willing to do that in the real world.
A couple is also more likely to find men willing to join them in any tech-based method than a woman willing to join them in a tech-based method.