Yes, you need all three.
So dust off your copy of ruby/sapphire/emerald and get working.
nope cause all the other reggies will awake reggigas from his sleep.
to get reggigas you need to have the "special reggigas" which i think is an event pokemon, then you go to get "regirock, registeel and regice" then go to snowpiont city, and go to the temple (if you have beaten the elite four) and keep going down and find your way to reggigas.
you get reggigas when you get reggirock, reggisteal, and reggice and go back to snow point temple to the reggigas statue and battle him
After you get all three Reggies (Regice, Registeel and Regirock) there is not much else to do with them seeing as you most likely caught them on the pokemon sapphire/ruby/emerald game. For either Pokemon diamond, pearl or platinum you can trade the three reggies onto it and go to the sacred chamber place which is the town below the snowpoint city gym and with all three reggies in your inventory you can unlock an entrance which leads to Regigas (sorry if spelt wrong been a while since played) it is very powerful and is level 70.
You need to capture Regice, Regorock, and Registeel and then take them to the Snowpoint temple where the Regigigas statue is, then you fight it. But I can't remember where you get the three 'reggies' as i like to call them.
No.
The 3 reggies are not in platinum they are only in the hoenn region unless you use cheats.
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yes
Battle him a second time.
Well There is a lot if you know dive and surf. Like wailmer evolved or wailmer which u need to get reggies.
reggigigas is in a temple and is guarded by three pillers.one is steel one is rock and one is ice.reggi rock,steel,and ice are caught by the temple master named Brandan.