How do you get the three regis in Pokemon HeartGold?
Migrate from Pokemon ruby emerald Sapphire
Actually you're wrong you can NOT get the regis in Pokemon diamond, to get the regis you need Pokemon ruby/Sapphire/emerald you can get all three regis there(regi ice, regi steel, regi rock) althought if you migrate all of the regis onto Pokemon diamond you can get regi gigas(king regi) you will find him in the snowpoint temple after beating the elite four. Thanks hope i helped ^.^
You need relicanth and wailord to catch the regis in sapphire/ruby/emerald. You cannot catch the three regis in platinum unless you have the event Regigigas (this event has now ended). If you have the special regigigas in your party, three new caves will open up in Sinnoh and you will be able to catch the three regis.
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 regigigas to awaken the three regis.so you need to get the three regis from a gameboy advance Pokemon game.