I don't know what or if any law requires a motion in PA, but in Utah all you have to do is talk to your Probation Officer and ask him if he has any objections for you to get off probation early if he does not than he will or can go before the Judge and ask him. Usually on a Felony 3-year probation you will only have to do it for 1 year, that is if you have completed all of your requirments like drug classes or community service, fines etc.
I hope this helps you good luck.