For Nintendo Wii there is no such thing as downloading guitar hero 3 tracks through official download services.
However, there are a few alternatives:
- If you have a modded wii, you can find yourself edited versions of guitar hero 3 (for example, I have versions with songs from guitar hero 1, 2, 80's, rock band, moar rock band track versions) through things such as torrent or newsgroups.
- If you do not have a modded wii, you can still choose to do the twilight hack and go for the homebrew guitar program, which uses frets on fire tracks. It has it's own topic somewhere on this forum.
- If you have an unmodded wii, you can still choose to use the homebrew way of loading backups, thus allowing you to still play the modified versions of guitar hero III available on the web.