Is it legal to marry my usa fiancé on a tourist visa. My boyfriend is currently in usa penitentiary and may be released in 3 yrs if he gets clemency. Can we marry then apply after for me to stay in the usa

First you will have to get permission from the penitentiary to marry, they will be strict on rules and paperwork.
So I doubt the prison will allow it to happen, even with a marriage license since you don't have the proper visa.

Next if they do, merely getting married does not get you the proper visa automatically. So you most likely will still have to leave and apply for a new visa to re-enter

It's not that easy, he may not be qualified to sponsor spouse visa for you. There are many other factors involved. Consult immigration attorney and see if it is worth waiting for 3 years or simply move on.