I know Diablo 3 requires Battle.Net client to download/update/install
But I have never used Battle.Net
Will it automaticaly recognized the game files if you were to move them to a backup drive and restore them later?
I've put many hours into Path of Exile and never tried backing up the files and installing them later on.
One thing you "can" do is backup the virtual drive for each game. You can copy the entire virtual drive except for the "dosdevices" folder in the virtual drive root. This is because there are links to your home folder and othe folders.
What I do is move the "dosdevices" folder from the virtual drive game folder, then copy it to my server/backup drive, then move it back. But when you want to restore it, you have to create the virtual drive again and then copy all of the files back into drive_c