Had many casual MFF or MMF threesome (and moresomes) in the swinging scene. Always good fun when everyone clicks, if not working can easily call time and move on in that setting (as opposed to a prearranged threesome meet up or W/L threesome session where "early" ending can cause significant tensions/awkwardness or there is a reluctance to call time.)
Most memorable was one Sunday afternoon session at a swingers venue - had arranged to go with a regular FWB as a couple with no agenda but to go with the flow & enjoy whatever presented itself.
On way, she casually told me she she was talking to a girlfriend she'd not seen in a while and mentioned our plans for the avo. She casually indicated that the girlfriend (who had been out of the swinging scene for a while) had been intrigued so was going to go to the club as well to see what was on offer to put a toe back in the water. Turns out my FWB had actually invited her to come specifically for a MFF threesome as a treat for me (as she revealed on the trip home!).
What a wild afternoon the three of us had - lots of fantasies and few boundaries. Lots of voyeurs watched (and would have been welcome to join) but chose to leave it to me to enjoy and pleasure these two greedy girls.
We have since repeated several times, always good times but not as spontaneously erotic. We also regularly enjoy threesome (and moresomes) either prearranged or casual at swingers events, without damage to our open FWB/relationship. Transparency, communication, mutual respect are essential for this to work.
Have also recently been invited to assist a tantric masseur give a 4 handed Yoni massage to the female of a couple where the male wants to treat his (very willing and horny) partner to a sensual experience and watch rather than participate in the massage (and whatever ending ..........).