Blizzard has added a timeline of recent events in the Diablo games series to their D3 website. There’s nothing new about the plot past the conclusion of Diablo 2, but this does confirm the consensus perception of the teaser shots from the D3 cinematics: This is Baal’s army massed to lay siege to the Barbarians’ citadel (a larger city than Harrogath), then decimated by the explosion of Mt. Arreat.
The Dark Wanderer completes his transformation into Diablo; Mephisto is destroyed beneath the jungles of Kurast by the band of heroes.
Diablo is defeated. His Soulstone is destroyed, along with Mephisto’s, upon the Hellforge.
Baal and his army march on Mount Arreat.
The heroes follow Baal to Mount Arreat for the final conflict between man and the last of the Prime Evils. Although the heroes defeat Baal, his demonic army continues to advance.
The angel Tyrael discovers that Baal has succeeded in corrupting the Worldstone housed within Mount Arreat. Realizing that the Worldstone cannot be restored, Tyrael reluctantly destroys it. The resulting explosion destroys Mount Arreat, ravages much of the countryside, and decimates Baal’s army.