How do you awaken Regigigas?

Yes, you do need all 3 Regis (Regice, Regirock and Registeel) in order to awaken Regigigas in the Snowpoint Temple. They must be with you when you awaken Regigigas.but then again it says you need...

You need Rigice, Registeel, and Regirock in your party, then talk to Regigigas.

In Pokemon diamond?pearl, you must get 1 of each of the other regi's in your party and get to his statue un the snowpoint temple. He's lvl 70, so beware if your Pokemon aren't high lvl.

you need regice registeel and regirock then go to the snowpoint temple

deep in the snowpoint temple of snowpoint city