After you've caught all of the Regi trio, you can take them to the basement of Twist Mountain, through a cave past where the stone is that lets you evolve Eevee into Glaceon. Yeah, you know the place.
Anyway, assuming you have the three Regis in your party, you'll be able to awaken Regigigas and have a chance to catch it!
|Regigigas Lv. 68
Requires Regirock, Regice, and Registeel