That depends. In the Sinnoh region, you can try that temple in Snowpoint. it has the 4th regi, Regigigas. If you're playing ruby/sapphire/emerald, then nothing.
you only can find him in snowpoint temple in snowpoint city but you must have all three regis. bring all three regis go up to him and press A and the three regis will awekening him. hes a level 1 so have a goodb time catching him.
nothing. you catch them. what more can you ask then the regis'? don't be selfish! and you cant catch regigigas.
You Can't
You can't find all the regis (without the use of an event Regigigas) in Platinum but you can find Regigigas and you need Regirock, Regice and Registeel to enter the Snowpoint Temple and if you have action replay you can try catching the 3 regis too.
nothing will happen but i recommend catching it because you need it to catch the regis
you cant get all the regis only regigigas.
become champion or have all three regis, you can only get regigigas if you have all three regis
Use a cheating device. You can't get 99 masterballs on sapphire after catching the three regis without a cheating device. Also you din't need any regis to have 99 masterballs.
u can find him in snowpoint temple after catching the 3 regis.u cant get the 3 regis unless u migrate them.
All 3 of the Regis are level 40.
Catch them all.
well if you want to get all of the regis 1 way u can migrate them from ruby or sapphire