Little is currently known about Act III. It largely takes place in the Barbarian highlands, in the far North of Sanctuary. The region includes Mount Arreat and the Arreat Crater - the location where the Worldstone was destroyed in Act V of Diablo II: Lord of Destruction.
It's expected that the player will chiefly be tasked with defending Bastion's Keep - one of the few remaining Keeps that were designed to defend the Barbarian lands - from the demonic invasion, alongside other human defenders.
Bastion's Keep concept art.