Migrate from Pokemon ruby emerald Sapphire
You cannot catch any of the Regis (Regirock, Regice, Registeel and Regigigas) in Pokémon HeartGold or SoulSilver.
Regigigas cannot be caught in Heartgold version. To get a Regigigas in Heartgold, you would have to trade it from Diamond, Pearl, or Platinum. To get it in one of those games, you have to transfer all three Regis (Regice, Registeel, and Regirock) from Ruby, Sapphire, or Emerald.
you can't get the three regis but, if you have pokemon sapir\rubi\emerald version on the gameboy advance, you can tke the three regis through the pal park in route 221 (in south). *there are three caves that may be the caves of the three regis but i don't know what to do there.
trade them
trade from emerald
No, they will need to be traded.
For Pokemon ruby you can catch the three regis, rayquaza, groudon, and latios/latias. In sapphire you can catch the three regis, rayquaza, kyogre, and latios/latias. However, in emerald you can catch kyogre, groudon, rayquaza, the three regis, and latios/latias.
You can not capture any of the regis in Pokemon pearl, diamon, or platinum you have to migrate from ruby, sapphire, or emerald. or trade with someone who has them.
you don't
Chickorita, Totodile, and Cyndaquil.
no you do not just the Pokemon relicanth and wailord.
You need the event regigigas to obtain the three regis or the three regis to obtain the game regigigas, its a cycle unless pal park, trading, cheats or events are used.