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:
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. Guitar Hero III: Legends of Rock Free Full Game...
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. If you are looking for the "full game"
This is considered the definitive way to play Guitar Hero 3 on PC. It is a modded version that fixes common bugs, improves performance, and is often distributed as a pre-packaged installer. Guitar Hero III: Legends of Rock Free Full Game...