I Bought The RCT3 Platinum In the steam but i don't know if mods work on steam or pirated versions?
I Am Not sure if RCT3 Platinum Steam version needs downgrade or not?
I Tried to search it on youtube or tried to google it and i don't know wich one is?
Please Answer it!

There should be no problem using mods through the steam version. Are you trying to add custom scenery?

Yes I Added Custom Scenery Into the game in the Steam version when i clicked on the custom scenery it worked it works in both versions.

I've never had issues with Custom Scenery on my version.