It is usually not that tough. You don't want to unscrew those three screws. Most modern units are removed (after disconnecting the power and drain plumbing) by simply turning the large collar at the top of the disposal about 1/4 of a turn to the left as you are facing the sink. I find it best to sit where you can stick one knee under the disposal to help lift it slightly while you turn the release ring, then it should just remove straight down. Good luck.