It was announced in a tweet by the official Pokémon Twitter that the trio of Legendary birds Articuno, Zapdos and Moltres will be distributed to fans at random in May.
The three Legendary bird Pokémon made their first appearance in Pokémon Red and Blue (Red and Green in Japan). This time around, each one will come with a unique Hidden Ability. Articuno will have the Snow Cloak ability, Zapdos will carry the Static ability, and Moltres will come with the Flame Body ability.
The only way to get one of these Legendary Pokémon is to sign up for the official Pokémon Trainer Club Newsletter. In May, any member who signs up will be given a code at random to download one of the three Legendary Pokémon.
To qualify for this offer, you must own a copy of Pokémon X/Y or Omega Ruby/Alpha Sapphire and be signed up for the newsletter by April 22nd.
At the moment, it appears that this offer is currently only available in North America, but Pokémon fans living in Spain are in luck! If you live in Spain you can head to your local GAME store and receive a unique serial code for the same offer.