Cheating is cheating, one thing leads to another, you two are probably over. ( I was in the same situation 15 years ago) if it hurt in your gut then it's cheating period.
My advice, confront her, get your threesome fantasies out of the way, do things most men only dream of, grudge fuck them both, and divorce her.
You will find a faithful one down the road. She is a slut so treat her like one, sow.your wild oats and take care of your needs and get out. Guaranteed she is a sneaky cunt and this is not the only place she is fucking you.
Guaranteed if you were with a man she would call it cheating

Good luck!