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.

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

A massive overhaul that adds new songs, characters, and menus, available for both PC and PS3 (via emulator) . 2. Modern Alternatives

A massive mod for the World Tour PC port that allows you to import the entire GH3 setlist and play with improved graphics and features. 3. Legacy Downloads