Because the game is no longer sold digitally, it is often categorized as "abandonware."

The community has largely moved toward open-source engines that are and support all songs from the original Guitar Hero series:

While there is no official "free" version of Guitar Hero III: Legends of Rock from Activision, you can access the game or its content through several safe, community-driven methods for PC.

This is the most popular modern successor. You can download the engine for free at clonehero.net and then add "song packs" that contain the entire Guitar Hero III setlist.

If you are looking for the "full game" experience with modern improvements, fans have created standalone installers that simplify the setup on modern Windows systems: