Regigigas
Normally, you need to have the three golems Regirock, Regice and Registeel to awaken Regigigas in Snowpoint Temple. Paradoxically, however, you first need Regigigas to spawn the golems. Once you decide to cheat your way through, enter the code into your Action Replay along with the Game ID "CPUE d074D1B3 3811DEF6" for "Pokemon Platinum." If you are using an emulator, you will not need to enter the Game ID. Next, press "L" and "R" at the same time to spawn the character that gives you the legendary Pokemon. You can find him in any Pokemart wearing a green outfit. Simply talk to him to receive your reward.
Regirock
Travel to Route 228 with Regigigas in your party, and look for the entrance to a cave. In the past, this cave was mostly empty, but thanks to the presence of Regigigas, the secrets of the cave are unlocked. You will notice a series of dots on the floor and a statue at the end of the room. Step on the dots and walk up to the statue to trigger the battle with Regirock, one of the three legendary golems. He is a level 30 rock-type Pokemon and will not be difficult to defeat.
Regice
For the next golem in the set, revisit the cave on Mt. Coronet near the exit to Route 216. You will enter a new cave instead of the old empty one you saw on your first trip through earlier in the game. Once again, step on the pattern of dots in the middle of the room and approach the statue to encounter Regice, the ice-type legendary golem. Like Regirock, he is level 30 and is easy to capture.
Registeel
To complete your trio, take the boat from Canalave City to Iron Island, and enter the cave near the exit. Yet again, you will find a pattern of dots and a statue instead of the old empty cave. Walk to the statue after stepping on the dots to spawn Registeel. As he is only a level 30 steel-type, you will not have trouble capturing him.