About 3 years ago I gave my sister who lives in upper Wisconsin temporary custody of my son. I was young and stupid and finally came to my senses and got myself back on my feet about 2 years ago. My sister now hasn't really allowed me much contact with him but I wish to resume my role as his mom. I was just wondering how about starting the process of getting custody back?

I doubt there is three years TEMPORARY custody order. I think your sister got a permanent one.If she has the custody-she has right to decline your visitation.
You should get an attorney and file ASAP.You should be aware that a court will not want to make an immediate change