In Diamond & Pearl, when you transferred Regirock, Regice & Registeel over to the games through Pal Park, you would then have access to getting Regigigas in Snowpoint Temple. Howver, Platinum has things the other way round...

From June 20th to August 31st in Japan, if you pre-order your ticket for the 11th movie in Japan, or gone to a Toys 'R' Us event in America, you'd be given a special Regigigas. If, when Platinum is released, you send the Regigigas over, a special event will activate within the game. Only the special Regigigas given away will activate this event.

At a certain point in the game, certain new areas will crop up in Sinnoh, these include caves that hold Regirock, Regice & Registeel. They are all at level 30 and you need the movie Regigigas in your team. However, this does not prevent you from taking other Pokémon considering the massive level difference. All you need to do is awaken them. Below is a list of them as well as where their caves are

#377 Regirock

Location: Route 228

Lv. 30

#378 Regice

Location: Mt. Coronet Route 216 Exit

Lv. 30

#379 Registeel

Location: Iron Island

Lv. 30