No cheat code unlocks "Free Bird" in Guitar Hero III: Legends of Rock on PS2 (or any platform).
Complete Career Mode on any difficulty (Easy through Expert).
On Wii, you *can* enter gesture codes at the main menu (e.g., "Unlock All Songs" = specific swing pattern). But "Free Bird" still requires Career completion even with "Unlock All" active on some versions.
Beat the devil, earn the bird. That's the only way.