In Omega Ruby, you find Tornadus in a storm cloud in the sky while souring through the sky with a castform on your team, then you will find Thundurus in Alpha sapphire (With the same requirements) so you need to trade one over, then with both of these Pokemon in your party, if you sour the sky again, then you will be able to encounter Landorus. Another way to obtain these Pokemon is through transferring them from Black and White, or Black and white 2.
