After you have landed yourself a couple Legendary Trios (like Palkia, Dialga and Giratina and Uxie, Mesprit and Azelf) in Pokémon Omega Ruby and Alpha Sapphire, you might as well catch the rest — right?
The Legendary Tao trio (also known as the Tao Dragon Trio and Energy Trio) are formed by Reshiram [Dragon / Fire type], Zekrom [Dragon / Electric type] and Kyurem [Dragon / Ice]. In order to catch the trio you will need to trade or transfer Pokémon, a level 100 Pokémon and Soar (not to mention plenty of HP restoring items and Poké Balls).
Soaring near Mauville City with a level 100 in your party (if you do not have a level 100 in your party you can trade or transfer one from a previous game) will cause a new area to appear called Fabled Cave. In this cave you will encounter either Reshiram or Zekrom, what Pokémon you encounter depends entirely on whether you have Omega Ruby or Alpha Sapphire.
- Omega Ruby: Reshiram
- Alpha Sapphire: Zekrom
Once you capture on of the two, you are one step closer to capturing Kyurem. The next step would be getting your hands on the Reshiram or Zekrom you didn't get from Soaring into the Fabled Cave. You can go about this by transferring the Pokémon from Pokémon Black (1 or 2) and White (1 or 2).
Once you have Reshiram and Zekrom in your party take back to the skies and Soar near Mauville City and instead of the Fabled Cave you will the Gnarled Den. In the Gnarled Den you will find a rin that leads you to Kyurem.
*You should make sure to bring your Dowsing Machine along, if you use it in the cave where Kyurem is you will find DNA Splicers that will allow you to create Black Kyurem or White Kyurem by fusing Reshiram or Zekrom with Kyurem.
Good luck!
Here's where you can find more guides for catching legendary Pokémon and finding Mega Stones.
