Three years. That is a long time to invest in a man. I take it you didn't realize he was married until, what, last week?
I hope that there are no children involved, that he doesn't leave your bed, and go home and set a good example of what a man and father is to his wife and children.
Did you ever think that part of the reason he sees you, is that he has become distant to his wife (and family), and silence is covering up guilt and remorse? While you service him with what is missing in his life, his wife has no idea she's being stabbed in the back by a dishonourable woman? Really- what kind of woman does this to another woman.
He, instead of being a man, stepping up, trying to work on his marriage, falls for a woman who is easily available for sex. It's worked for three years, and yes, you have been used. But, you allowed yourself to be used.
It seems obvious he's not going to leave his wife. That's too bad because she could probably be happier divorced from him. Too bad for you as well, because when you eventually wake up and realize your life with a married man is going nowhere, you've wasted a good chunk of your life.
I don't know what kind of advice you are looking for here. If you are wondering what you should do, my advice is to drop the married man, show a little belated respect for the other woman (wife) involved here, and possibly an entire family, if there are kids, and get out.
What you have is not love. Not even close to an honest relationship. It is all based on deceipt. Who cares if he's seeing someone else as you suggest- for all you know he's seeing his wife, and she's NOT sleeping on the couch.
Get a single, available man, and leave married men alone.