The charge for each extra hour on a typical car rental is 1/3 of the charge for an extra day. Most car rental companies have a grace period, so you can return the car slightly late (say, 29 minutes) before being charged, but otherwise you'll be hit with the relatively high extra-hour charges that are specified in your contract.

What this means is that, for a typical rental, after keeping the car for 3 extra hours, you're essentially paying for an extra day, and the full extra day charge applies. That's just the way it works for the majors.