I wonder if the item is an attachment item or just a item like for shaymin (I hope it is the latter)
They got really nice abilities:
Tornadus- Regenerator- one of the best abilities on offensive and defensive pokemons a like.
Thunderus- Volt Absorb- immunity to a type you don't resist is always great
Landorus-Intimidate- Intimidate is an awesome ability for set up I don't remmember if it gets swords dance but if it does then it is a really good one.
Correct me if I'm wrong, but Thunderus was banned because of his capabilities to use Prankster Twave and cripple anyone who can counter him, as well as giving him a turn to set up. His new ability will do the opposite of that; he'll for sure stay in OU, but will be more easily defeated by Scarfers. Also, he's going to out-class Jolteon in every way and just push the little cat-thing down to UU/BL. He already out-classed Zapdos as a sweeper, and Zapdos doesn't even have Volt Absorb yet.
As for Landorus, Intimidate
does make him bulkier, but part of what made him so successful was his ability, Sand Force, making his STAB move intensely powerful and his Edgequake coverage really hard to match. Intimidate makes him
worse, if anything, because Gyarados and Salamence both have Intimidate, are bulkier (at least in Gyarados' case), and have a much better move-pool. Yes, they don't get STAB Earthquake, but they have access to other moves that gives them really good coverage.
Tornadus is the only one who will get a sort of boost. Regenerator, along with the announced bulk increase, allow for him to be a wall or have some support role. This is radically different from the current use of Tornadus, who is purely a special rain sweeper. His typing isn't great, but Regenerator kind of cancels out his weakness to SR.
Of course, it also depends on move-tutors, other DW pokemon that will be released, and the release of the final three legendaries and Keldeo's new form.