I know you have to have the three og regis in party, can they only be accessed through the event that happened a while ago or trading? Thanks, and if not, is there a specific way I can get them without trading or the event that already passed? Thanks!
There was an event in which you could get a Regigigas and access the Regi temples and capture them to get to Regigigas. However, that ended a long time ago.
Transfer from Hoenn
You can transfer the regis to Sinnoh via the Pal Park. This lets you capture the Regigigas in the Snowpoint Temple.
All Regigigas Events in Platinum have ended.
As a result, the only legitimate way to get Regirock, Registeel and Regice is by trading from RSE.
You can cheat Regigigas into Platinum to get the Regis, so here is a guide at the end of my answer. It requires an emulator. Anyway, here is how to get Regigigas in Platinum.
1. Have all 3 Regis in your party in RSE and put RSE into the DS. Then put Platinum into the DS slot and start it. Select migrate Pokemon from RSE. You will have to complete the Sinnoh National Dex. You also need 6 Pokemon in your party in RSE to do this.
2. Go to the Pal Park in Platinum and start the catching show in Pal Park.
3. Walk in the grass until you find all 3 Regis and the other 3 Pokemon.
4. Have the Regis in your party and go to Snowpoint Temple.
5. Go to the bottom of the Temple and you will find Regigigas. Make sure to save before battling Regigigas before battling it as you only have one shot.
Source, it also contains a cheating guide: https://www.wikihow.com/Catch-Regigigas-&-the-Hoen-Golems-in-Pokémon-Platinum