Legally, yes. Activision still owns the copyright to the code, the engine, and the master tracks. Even if they don't sell it, distributing it on SteamUnlocked is copyright infringement.
Guitar Hero III: Legends of Rock can often be found on third-party sites like SteamUnlocked, it is important to note that the game has been officially delisted from most digital storefronts due to licensing expiration. Game Overview & Content guitar hero 3 steamunlocked
: You may need third-party software like JoyToKey or X360CE to get specialized guitar controllers to register correctly. Legally, yes