There will be no Diablo 3, because the producer of Diablo 1 and 2 ( Blizzard North Team ) were fired after working 3 years on Diablo 3.
About 1 year later they developed a new firm and started to work on their own project.
Blizzard has still the right for the gamename "Diablo 3" so they had to think about their own name for the game.
In the end the ex-blizzard north team started to think about a "action mmorpg adventure".
This project has a cloased beta already, is based on nice 3D graphix with their own engine.
This project is the famous " Hellgate: London " Project guys.
So now you know the background story.
Here is a link :
http://www.hellgatelondon.com/
Regards Ruin