We've been to Gettysburg several times and have always had a great time.
Start with the visitor's center. There are movies, a great display of the battle and the cyclorama. There are usually various talks by Rangers at different stations around the center.
We took the auto tour which is great. I'm not sure what the tolerance for this will be with a 3 yo though.
Stroll down Main Street. Theres plenty of historic places and shops to pop into. Have an ice cream at the canon ball ice cream shop, so named for the canon ball still sticking from e side of the building.
Visit Eisenhowers estate, it's very interesting.
See if there is a ghost tour geared towards youngsters.