It's not necessary to play the previous games which I think was a smart move. There's even a museum in the game that tells you about the history of the entire outbreak so you won't be in the dark. --- I slept in too late, now I have less time to do nothing.
It would be hard to go back and play the first two games after playing DR3. The games were great but this one is superior in every way (Except for the main protagonist since most DR vets prefer Frank West and/or Chuck Green) --- GT - The Grunge Guy