So I have saves from my disc copy of RCT3...

[Maddie Gaming]

Any idea on how to transfer these saves to the Steam copy I have?

[Hopper]

If they are on the same computer, they should show up automaticaly in the steam version as save files are saved in the same location.

[Snicelory]

Did you get it, even with that not very helpful comment?
Just wanting to help, if you had problems :D