I have a Remington 870 with a deerslayer barrel that we shoot 3" slugs through, and your right is does kick like a mule, but that doesn't mean you can't be accurate with it. I know it is tough to admit, but you are probably jerking the trigger because of the recoil. Last year my younger step brother was going to be using that gun and he had the same problem. His groups were horrible. So what I did was i put an empty shell in the chamber and handed him the gun. Well, he pulled that trigger so hard you could see why his groups were bad. After that we made him slowly squeeze the trigger and his groups were good again.

You are definitely correct that you don't need 3" slugs to take down a deer, but to say that the gun isn't accurate just because of the recoil isn't really true. By the time the recoil moves the gun any signifigant amount, that slug is already flying down range.

For a good exercise, have someone else load the gun, and put in 2 spent shells and one live round. Do this for a while and it will get you more used to squeezing the trigger instead of pulling it.