I have done MMF and FFM threesomes. In every case one party brough up that they were interested in doing so and the rest agreed. In all cases we had some level of friendship.
One case the tree of us worked together, another I was dating him, worked with her, another fwb with her and open relationahip with him, another I was married and he approached her, another I was married and we alproached her, another dh & bf set it up for the night prior to a major surgery I had as a gift to me. In all cases both men were straight. All the women were either bi or lesbian.
"Love As Thou Wilt"