Theres two ways to get them, the first way you need to transfer the three Regis from R/S/E.
1. You need the three in your party and then go into Snowpoint temple. Then you go all they way to the bottom level you'll see a Regigigas statue. Go and talk to it with the three Regis in your party and the battle will start.
2. There was a Regigigas that was a giveaway event that let you catch Regice, Registeel and Regirock in the wild on Platinum. You needed to have it in your party for them to show up though.
I hope that helps to clear things up some.
"Don't walk behind me; I may not lead. Don't walk in front of me; I may not follow. Just walk beside me and be my friend."