You can get codes for the Legendary Pokémon trio – Articuno, Zapdos, and Moltres when you sign up for the Pokémon Trainer Club newsletter and in the May newsletter. The codes will unlock all three Pokémon in your Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X, or Pokémon Y game. Sign up by April 22nd to be eligible.