First,You need to have Google Sketch up and RCT3 Sketch up Pulgs.
Download: http://dl.google.com/sketchup/GoogleSketchUpWEN.exe
How to use Sketch up?: http://www.google.com/support/sketchup/
Next, you need to have RCT3 Importer.
Download: http://www.sebar.com/jonwil/rct3imp15.zip
*All the file uploaded this post.
How to use RCT3 Importer?: You can fine at the video
Ok,did you downloaded?
Then, you can watch this video:
http://youtube.com/watch?v=DPF091WyJjk
did you watched the video? Finsh!!!
Put the rb file in "C:\Program Files\Google\Google SketchUp 6\Plugins" folder