Three is a hard age. ((Hug))
I always stand back and look at the surroundings and other things that might influence my child's behavior. Were they tired? Was it close to snack time? Were we in a new, exciting place? Were they bored? Usually there is something I can link it to and the next time I plan accordingly.
If I were a little one in a place new to me, with plenty of room to run and I was tired of waiting, I'd probably run away too. ;) It sounds like a mix of boredom + testing the waters.
Does he need to learn to stop and come when you ask? Absolutely!!! But it's not going to happen overnight. It will take gentle and patient training on your part.
Does he have a library card? Perhaps he would be excited to have a card and have the ability (with your help of course) to check his books out himself. While you are in the training stage, yes, the stroller is a great idea!!