From March 4th, 2016 until October 31, 2016, Pokémon Bank users that access their account can get Regirock, Regice, and Registeel.

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Regirock, Regice and Registeel available via Pokémon Bank

Players who access their Pokémon Bank between March 4 and October 31, 2016, will receive a Regirock, Regice, and Registeel that each possesses a Hidden Ability! These three Pokémon can be received using the Pokémon Link function in the Pokémon X, Pokémon Y, Pokémon Omega Ruby, or Pokémon Alpha Sapphire game that was used to access your Pokémon Bank. Note that Pokémon Bank requires an annual membership fee of $4.99.

You can get the Pokémon trio by selecting the Pokémon Link option from the menu screen of your Pokémon X, Pokémon Y, Pokémon Omega Ruby, or Pokémon Alpha Sapphire game. Once you've received the Pokémon in Pokémon Bank, select Pokémon Link and the three Pokémon will be sent to a Box in your game. The three Pokémon will be sent one at a time from Pokémon Bank. Note that you can receive these Pokémon only once per Pokémon Bank account, even if you have multiple copies of Pokémon X, Pokémon Y, Pokémon Omega Ruby, or Pokémon Alpha Sapphire.

Each of the three Pokémon that you receive has a Hidden Ability, a trait they do not possess when found in Pokémon Omega Ruby and Pokémon Alpha Sapphire. This is your first chance to get these Pokémon with their Hidden Abilities, so don't miss this opportunity!

Regirock, Regice and Registeel Details

Picture Pokémon Information
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Pokémon: Regirock
Ability: Sturdy
Moves: Explosion, Ice Punch, Stone Edge and Hammer Arm

With the Hidden Ability Sturdy, if Regirock has full HP and is hit with an attack that would normally knock it out, it will be able to endure the hit with 1 HP remaining. This is a great Ability for Regirock to have, being a Rock type with many weaknesses.
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Pokémon: Regice
Ability: Ice Body
Moves: Thunderbolt, Amnesia, Ice Beam and Hail

Regice's Hidden Ability Ice Body allows it to restore HP when the weather condition is hail. Regice is strong against opponents that attack it with special moves. By using Hail, it can recover HP throughout the battle while doing even more damage!
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Pokémon: Registeel
Ability: Light Metal
Moves: Iron Head, Rock Slide, Gravity and Iron Defense

Registeel's Hidden Ability Light Metal reduces its weight by half. This decreases the damage it will take from moves such as Low Kick and Grass Knot that are more powerful the heavier the target is. It is especially good for Low Kick, a Fighting-type move that is super effective against the Steel-type Registeel. Roll with those supereffective punches and strike back!

Sources: http://www.pokemon.com/us/pokemon-news/deposit-three-solid-pokemon-with-pokemon-bank/ and http://www.pokemon.co.jp/ex/pokemonbank/present2/