Will probably just stick with Prankster.
For Thundurus in VGC, yeah for the most part. But it will give him a way back into the OU tier, and it would give him opportunity to stay in against some Pokemon he'd flee from otherwise..
Tornadus could be really entertaining with Regenerator, one of the best abilities in the game. It's another Pokemon that can spam U-Turn now, recover off its 25% SR weaknesses easily, appears to be leaning toward defense with its new form and it benefits from Prankster much less than Thundurus. Again, mostly based around OU, where Regenerator won't see much play in VGC.
Landorus is actually the one that benefits from doubles the most, adding another Intimidate Pokemon to the mix. Sand Force is an amazing ability, but Intimidate lets him have access to an entire set of teammates and strategy.
Remember that they'll have whatever new moves B2/W2 gives them and new stats so you shouldn't make up your mind before all the info is in.
Tornadus will specialize in Defense, Special Defense, and Speed; Thundurus will specialize in Special Attack; and Landorus will specialize in Attack.
For the most part, the CoroCorco information looks like Thundurus and Landorus will still focus similarly, with Tornadus changing the most to a defensive minded Genie. Of course, we can only speculate until the game is out (probably), but it's fun to think about.