The old 3 inch casters are frozen. When I try to remove them with a long socket wrench, the bolt (threaded stem) just turns. There is no way to get a grip on the other end to prevent this. The space between the bottom of the cart and the wheel is too small to get a wrench on that side of the bolt.
Try cutting a groove in the head of the bolt to fit a large straight blade screw driver. I would use a dremel or other rotory tool. You may be able to reuse the bolt if you are careful.
Some manufacturers make specific wrenches to remove casters. They are basically flattened wrenches about 1/8" thick. If you have the manufacturer of your caster you can call our sales team to determine if there is a caster wrench available to help you out. 1.888.388.6372