The Wii version is unique because it utilized both the internal flash memory and SD cards.
Unzip the downloaded file. Inside, you should see a Saved Games folder containing files like GH3P1.sav . Drag that entire folder into C:\Users\[YourUsername]\AppData\Local\Aspyr\Guitar Hero III\ . guitar hero 3 save file
Rename your existing Saved Games folder to Saved Games_OLD . This way you can revert if something goes wrong. The Wii version is unique because it utilized