beat elite 4
you have to have them traded from ruby Sapphire or emerald
you cannot catch any regis in platinum you have to migrate them or trade for them but if you get regirock, regice, and registeel on platinum then you will be able to catch regigagas at level1 in the snow temple
You need a event Toys R Us Regigigas.
yes but only through the special Nintendo event
Yes, that is the only way.
No; you cannot catch the three Regis in Platinum, UNLESS you have obtained a special Regigigas through some special events.
no you can not
that doesnt make sense
you need REGIROCK, REGICE, AND REGISTEEL.
You can only catch Registeel in Platinum if you have the Japanese movie or Toys 'R' Us event Regigigas. Registeel is hidden inside a secret cave within Iron Island if you have the proper Regigigas to unlock it. The cave is near the exit. If you don't have the specific Regigigas then you have to migrate Registeel from Ruby/Sapphire/Emerald.
Registeel is not catch able in Pokemon LeafGreen. In order to get a Registeel you will need to trade for one from Pokemon Ruby or Sapphire.
You can't because regigigas is a 4th gen pokemon. If you want a regigigas then you will have to migrate Regice, registeel and regirock to Diamond, Platinum or pearl