I believe after those Soulstones were destroyed, they did not die, but returned to the Black Abyss (the relation between Black Abyss and the Burning Hells is not very clear, as mentioned in http://diablo.wikia.com/wiki/Black_Abyss), not being able to influence so directly in the course of Sanctuary, but eliminating the Worldstone - the barrier between Hells and Sanctuary.
The point that i don't get is:
Did the Lesser Evils were also aware of the masterplan orchestrated by the three Prime Evils?
Until the end of Diablo II it seemed the lesser ones also took part in the scheme, since both Andariel and Duriel helped Diablo and his brothers...
But Azmodan's intention, as pictured on the Blizzcon cinematics, seems to become himself a Prime Evil. It could be that the Three sacrificed a little of their power to destry the Worldstone, and in exchange would rank up the Lesser Evils (make them as strong as the Three) in exchange of helping them in the final assault over the mortal realm.
What happened to Belial, it's not clear either.
I don't necessarily agree with everything you say but you got my light bulb flickering. I think everyone should *assume* that Belial is up to something because:
a) he isn't mentioned hardly anywhere and
b) he's the lord of lies/deception
Now, since belial and azmodan were warring with each other, andariel and duriel ( all 4 of which are lesser evils ) joined up to "help" the 3 prime evils ( diablo, mephisto, and baal ).
Now, azmodan doesn't seem to be on anyone's side but his own. What if belial sent the other 2 lesser evils ( andariel and duriel ) to "help" the 3 prime evils in order to ultimately get them killed and sent back to hell where he could use them to defeat azmodan who appears to be dominating hell.
All belial needs is some kind of control over the 3 prime evils. What that control could be, who knows, but we should all know that pretty much all of the evils are back in hell right now.
I think Azmodan is simply the commanding force in hell right now but you need some kind of an item to actually be a "prime evil" since he's not a prime evil already, such as the black shard/stone or w/e it is.
I think the 3 prime evils + belial + 2 lesser evils ( duriel and andariel ) are possibly going to try to stop azmodan from controlling them all. I also think that belial is going to try to manipulate all of them to become a prime evil himself. And finally, I think that Mephisto knows exactly what's going on as well and the 3 prime evils are actually *allowing* belial to manipulate them in hell in order to crush both of them, take over hell, and get the black stone so that they can then take over the mortal realm and defeat sanctuary.