Diablo III wasn't designed to be played in third-person, and it can't be, but the above video is a fun example of what the game might have looked like if Blizzard had opted for that camera angle. The work of YouTuber monstrousd3, the footage looks surprisingly good, especially considering the maps and animations weren't designed to be seen from this point of view.
Hence why the video doesn't show the player-character moving. "I can move," monstrousd3 wrote in the video comments, "but I have to control the camera manually and my character actually moves faster than the camera resulting in some start/stop catchup."
The video was achieved by manually tweaking the game's camera angles. It's not a mod, and even if it were it probably wouldn't play very well. Still, it's fun to imagine what the Diablo world would like like as a third-person action game.