I have never done a menage a trois, either as a MMF or a FFM, but I have fantasized about it a few times in my life - last time just recently and just very briefly. In the FFM version of the fantasy, a close girlfriend of mine and a man I cared deeply about were involved, purely as a way of pleasuring the man because I cared, without lesbian anything. So if I fantasize about it a bit, I probably could pull it off. I find it a selfless fantasy, but I do not know if it can help an ailing marriage. I guess it probably can't... but I have no experience so I should not be saying anything.