What I learned about this process is that breastmilk is more concentrated and is more filling than formula. Formula is mostly water and they tend to pee a lot more and are still hungry. So it's okay to give him a couple more oz of formula if he's hungry. It won't hurt him. You also want to make sure he doesn't have gas. The bubbles will make him extremely uncomfortable. What I did with my baby was give her the gas drops and/or just massage her body gently to work out any gas bubbles. It relaxes them and they potentially will go to sleep giving you an extra few minutes to yourself.
As far as the every 3 hours of feeding. I could get 2 hours before she was hungry but all babies are different. It's his way of say, "Mommy I'm okay because I'm hungry and I'm eating". I hear horror storys on how some mothers can't get there babies to eat that often. Then I would worry.
Hope this helps a little.