I'm going to start by playing devil's advocate and ask some rhetorical questions for the sake of balancing out the thread.
What actually happened? What does "hot and heavy" mean?
Did it really only happen once on the spur of the moment, under the influence of alcohol? Did anything lead up to it and how have they interacted since, especially immediately after the event?
Since it was three years ago the likelihood is that they had guilt at the time, covered it over, and hoped they'd got away with it. Three years later, might remorse not exactly be the emotion they feel? Might it be more relief mingled with embarrassment, and a little anger directed to the one who betrayed their betrayal?
And back to LT-mode:
Some people can deal with this kind of revelation, some can't. Some shrug it off or get a little upset but for others it's a deal-breaker. Only you can decide what category you fall in. Regardles of your choice in this, you have every right to feel whatever way you do. You have been betrayed and no-one should denigrate you for that.
"Get over it" is not an acceptable response to someone who has had a recent revelation of betrayal, though I can understand that the natural response would be to want to minimise the situation because he doesn't want the flak that he is accountable for, nor any other potential consequences. Do you have kids?
How would your husband feel if the shoe was on the other foot and you had got drunk and had a passionate kissing session with a work colleague 4+ years ago? Would he really be cool about it, or would it niggle him that someone interfered with his "property rights"?
Whatever you do, don't attempt to exact revenge by doing something similar to even the score. You can never even things up and all you'll do is make yourself feel cheap. You do need to take time to think through how this makes you feel and what the consequences will be.