Instructions
Capture Regirock, Regice and Registeel in either the "Pokemon Sapphire," "Pokemon Ruby" or "Pokemon Emerald" Game Boy Advance "Pokemon" videogames. The exact method of unlocking each Regi's lair differs slightly between games, but their locations are consistent between versions. Regirock is found in the Desert Ruins, Regice hides on Route 105's Island Cave and Registeel is inside the Ancient Tomb on Route 120.
Connect the Game Boy Advance title you used to capture the Regis to the Game Boy Advance slot on the Nintendo DS system and start "Pokemon Diamond." Choose the "Migrate From" option on the main menu and select Regirock, Regice and Registeel to transfer them to "Pokemon Diamond."
Go to the Pal Park in "Pokemon Diamond" and start a catching challenge. Walk around the grassy areas and throw Park Balls at the Regis to recapture them.
Bring the three Regis to the entrance to Snowpoint Temple in Showpoint City. The temple's door will open for players with Regirock, Registeel and Regice in their party. Descend to the fifth basement level of the Snowpoint Temple to encounter and capture the mighty Normal-type Regigigas.