Well Diablo III cannot be named Diablo without actual Diablo being in the Diablo game And he is certainly in DIII since we see him in the teaser video, he pretty much look like the Balrog in The Lord of the Rings, which isn't a bad thing at all mind you. And indeed none of the Prime Evils are "dead" per se, they were just sent back to the burning Hell, to "kill" them, from what I gather means to banish their essence, their "spirit" to some sort of a void or a permanent trap similar to the one Baal was imprisoned in, at the Tal-Rasha tomb, until Diablo and Marius (thanks to the influence of Baal on a mere mortal soul) freed him.
There are two very simple possibilities in DIII concerning the Prime Evils: 1) The "gate" linking the burning Hell and the mortal plain of Sanctuary is fully open thanks to a failed gamble by Tyreal after blowing up the Worldstone which might well have been what Baal himself planned from the start, a very well orchestrated plot to force himself into the body of the only Archangel who dares intervening in the mortal affairs... or 2) The mortal heroes make their way to Hell once more and this time perhaps more deeply into it to find all three Prime Evils (if Baal is part of them, pretending the "spirit escaping into Tyreal" theory doesn't work), kill their original demonic forms and in the process via some sort of extremely powerful artifact or another process involving the forces of Heaven banishing their energies/spirits into some void made expressly for them.
I'd also like to see a return of Lilith and the introduction of Lucion, both Mephisto's offsprings (Lilith also being Andariel's mother), maybe they can play a serious role in helping the return of Mephisto in the mortal realm, but without a Soulstone involved I wouldn't see how... but possibilities are there, Blizzard have all the talents and after all the Diablo cannon is theirs, and they can modify it as they wish.