Mephisto is most likely gone cuz he was a scrub and got farmed 10,000,000 times by MF sorcs.
Baal blew up into 1,000 ghost heads and flew off to go talk to himself all day long about how he never expected to be killed since people would just farm his minions and leave him alone.
Diablo however was a bit trickier than his brothers. While most people believe his soulstone was destroyed, he had a back up plan. While sitting patiently in the depths of hell underneath the cathedral in diablo 1, he read all seven of the harry potter books (they were released a lot earlier in hell than on earth). He became a great fan of Voldemort and said "minions! i want a horcrux in case someone smashes my soul rock with a hammer someday!" so his minions went forth and invaded sanctum in search for a good horcrux to please diablo. but demons not being too smart ripped open the portal to hell in the depths of a church in a little sleepy down called tristram. not being able to turn any holy items into a good horcrux, they ventured forth and raided the town of tristram. demons also being very lazy merely ripped the leg off the first kid they found. when presented with the leg diablo said "fine whatever this leg shall be my secret horcrux" as he was over the harry potter books since he realized his spells were a lot more badass than Voldemort's and wasn't gonna get beaten by some seventeen year-old kid with no parents. Breaking off a shard of his soulstone he jabbed it into the leg and threw it into a dark corner in the tristram cathedral for safe keeping. The kid survived and even got a wooden peg leg. he lived a short and miserable life on the outskirts of town ripping adventurers off by overcharging them for mediocre magical items. Though diablo was eventually defeated (prob by some 17 year-old kid) and the cathedral raided, the boy's original leg was never found. Diablo eventually came back with his 2 brothers and messed up the world again. this time tristram was raided and destroyed and the boy killed. many adventurers hated the boy so much they went back to get the boy's peg leg, steal their gold back, leave the body there, and use the leg in demonic cow rituals. Diablo and his brothers were defeated, their fates each mentioned above. But unknown to the world, when all evil was defeated from Sanctum for the second time, a leg in a dark corner in the depths of the cathedral began to glow. It took many years but diablo channeled his soul into the forgotten leg and grew it into his new demonic body. He channeled so much power into creating his new body it caused a great explosion in the cathedral making a giant glowing crater through it (you can see this crater in beta gameplay videos). He exploded into the world again shouting "B!TC#E$ DON'T KNOW 'BOUT MY HORCRUXES!" Thus diablo walked the earth once again filling the cathedral with demons and skeleton kings before disappearing to his secret hideout.
So after much explanation, it is my belief that diablo was reborn via Wirt's original missing leg. and THAT my friend's, is how wirt's leg will be worked into diablo 3.