in sapphire and ruby you have to go to paciflog town and go to the bottum house and surf down and left and for platinum you need the regigigas event and for black you need the event. hope this helped (WARNING: it may be wrong beacuse you need a relicanth and wailord.
go to sealed cave
you need event regigigas
Regi Trio Latias Kyogre Rayquaza
Try bulbapedia.bulbagarden.net It should tell you how to get the Regi trio: Regirock, Registeel, and Regice.
To
No, they are not, despise many arguments. Regigigas IS a legendary, but not the rest of the Regi trio.
route 207
In your dreams.
There is a little hole in the rocks near the water. You will find the regi rock there.
In normal gameplay of Pokemon Diamond and Pearl, Regirock, Registeel, and Regice cannot be caught in the wild. You will need to send them to Diamond or Pearl by inserting Pokemon Ruby, Sapphire, or Emerald with the Regi Trio in your PC and transfer them to the Pal Park.
In the snowpoint temple you need the 3 regi's from ruby,sapphire and emerald to wake up regigigas.
Either have the event, which I'm guessing you didn't get, or bring the three Regi's into your game and go to Snowpoint Temple's lowest floor. There, you can battle Regigigas at level 1. The Regi trio are from Hoenn though, so you need to get Emerald, Ruby, or Sapphire. There's a walkthrough for them at any gaming site.