Can you get the three regis in FireRed?
No you cannot get the regis nor any hoenn Pokemon all must be traded to you from ruby, sapphire and emerald.
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.