The Ressurection of the Three Prime Evils

[minicow]

What is actually going to happen:

-----Mephisto-----
As I heard at the Arreat Summit, Mephisto divided his Soulstone into seven pieces and gave one to each of his most entrusted Council Members. Sankekur got the biggest one which eventually became Mephisto's Incarnation. Theory complete. Mephisto's soul still roams Sanctuary in order to find a body worthy to corrupt.

-----Diablo-----
Unknown =D... but he is the main character of this game so he will be alive.

-----Baal-----
We never heard of his Soulstone being destroyed, but it may have fallen in that gigantic hole along with the shattered Worldstone. This means that it is almost impossible to destroy his Soulstone unless you have a kind of Soulstone Magnet. Baal has the same boring mission as Mephisto to find a worthy body.

[Saint Anger]

I might be wrong, but was it not the Worldstone that kept potent entities from heaven and hell from "freely" (easily) entering Sanctuary? So now, with its destruction, this should make it easier for both factions to enter Sanctuary, without having to bother with 'bodies' and 'soulstones' and whatnot.

Mephisto's soulstone was smashed at the Hellforge - there is no doubt about that.
There is a debate about Diablo's soulstone: a cutscene in act4 shows it being smashed, but some people point out as this cutscene is from Marius' perspective that this is mere hearsay and so it might have survived.
Baal's soulstone was broken in the events before Diablo 2, which is why the fragment that remained was not enough to bind him fully. That's why they needed a powerful mage - Tal Rasha - along with Baal's fragment to be able to fully contain him.

When you fight Diablo and Baal, their physical manifestations die - but their immortal essence, their "soul" so-to-speak, does not. You can clearly see it escaping from their bodies as they die. I'm not sure if it's that visible with Mephisto.

In all, I'm not convinced either Mephisto or Baal would make a comeback - we've been there, done that. Instead it seems the two 'missing' lesser evils - Asmodan and Belial - will make an appearance. Diablo seems to be a given, as its pretty much the titular character.

[Thirty-Thirty]

I tire of repeatedly discussing the return of the Three. It's complicated by what the soulstones did, where their spirits went (presuming they aren't dead), and what restrictions the Worldstone caused.

Basically, whereas other aspects find themselves with more or less evidence in one direction, anyone can comfortably believe whatever they want about the status of the Three. Asmodan and Belial are almost certainly in (according to an interview with a Blizz employee).

The only thing I'd say is that it'll pretty hard for fans to stomach a Diablo 3 in which Big D - in some form - is not the final boss.

[pantherus]

In Cain's journal he states that they destroyed Diablo's soulstone.

I think they'll try to work some story of the Worldstone's shattering enabling Diablo back...