It is hard to imagine that someone could "hide" who they are for 3 years. . .but then again, I married a guy that was completely different from the guy I dated for 3 years. He wasn't a criminal, at least not seriously (is poaching deer serious? :), but he hid his dark side very well.
Look at it this way, be thankful that you are able to avoid a very heart-wrenching future with a criminal. If you don't have children together, be EXTREMELY thankful.
I know the shock is a bit to deal with, but it WILL get better.
I've always heard about the so-called rule. . .give yourself a month of grieving for every year of the relationship. It took a bit more for me and my 13 year relationship, but not too mcuh.
For those with kids, the rule should really say. . . give yourself 1 month of grieving for very year or the relationship and 6 months of grieving for every child involved.
Good luck to you. As I said, count your blessings, give it some time, and move on.
Dyslexic agnostics don't believe in Dog.