Several menage books later I'm asking my readers this question. I think the answer is: As possible as any other relationship can be. But since I usually write fantasy menages, there's often something bigger than their personal feelings on the table. Some prophecy needs to be fulfilled, the fate of the world looms dark as they make their decision, or the death of one beloved member is at stake. Life and death or world destruction ups the ante, and if the three people involved can come to terms with more is better, then the logistics and petty jealousy must be put aside for the greater good.
In my present work I'm letting the characters determine their outcome. No matter what happens with the plot, they'll still have to decide if their relationships with each other will allow for a menage ending. They haven't come to terms with the solution ... YET. I need to give them a little incentive.
Do you think three's a crowd?