Well, at this point, Diablo 3 has not been officially announced, but there has been lots of evidence that supports its current development by the company.
There is also rumors that it might be officially announced on June 28th in Paris.
Here are some links Diablo 3 History Diabloii.Net Diablo 3 Forum
I really think that this might be it since Blizzard has already made or is officially making sequels or spiritual ones at least to its Starcraft and Warcraft franchises leaving just Diablo left of the three powerhouses.
Edit:
Okay...I was really stupid and posted this in the Gameroom. Could a moderator please move this to the Arena?
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