How do you go about getting a misdemeanor expunged since it has been 3 years and there are no other charges on record?

You will need to wait two more years. The statute says five years from your discharge from probation.

Start at an attorneys office with a full exposition of the facts.

There are only a few types of misdemeanors that qualify for expungement (your question doesn't specify which misdemeanor you were convicted of), and only if at least 10 years have passed since you completed your imprisonment, probation, or parole for the conviction without having any other misdemeanors or felonies during that 10-year time period. After you have completed the 10-year period, you can file a petition for expungement without assistance of an attorney.