I have played RCT 3 for a bit but I've always used the sandbox mode. Ive never tried the scenario editor.What is that? What's the difference between the sandbox and the scenario editor? Can you use the scenario editor to just make your own park free of money, just like the sandbox, or is the editor for something else?

the editor is basicly, designing your own game.

the "campaign mode" is were you complete challanges already set by the game. scenario editor is just making you own campaign

I have always used Sandbox. So much more challenging in my opinion. You do not have to worry about money and targets. You are free to let your creativity go wild.

I, personally, like the sandbox better. But, I have uploaded 2 scenarios as of right now!
Check them out and download them!