The bulk of my erotic writing has been MC, though I am now in the midst of a more overtly BDSM work. MC is certainly about the person yielding control, because they are generally mentally changed so that they want to do the things the controller desires of them, where as BDSM tends to be more about forcing them to do those things through various methods. For me MC is also about personality change, usually the person ends up with a different name and sexual tastes, e.g. an assertive woman with normal names goes from being straight and wearing plain clothes to wanting to be a latex-wearing, lesbian submissive with just a collar name.

Maybe in your case, the MC would be what would start you taking an interest in the other woman for the threesome, and her too. Maybe the other woman is a friend, that he would like to control too, or maybe a complete stranger that you pick up somewhere. Threesomes can be dangerous for long-term relationships, so the MC element can bring in a bit of a fuse, as it is not really you who is getting off with the third party because you are under MC (which can be temporary or permanent in stories) which you could be later released from, returning you to the status quo ante.