If you already own the game files (e.g., from an old CD or a digital purchase on GOG or Steam), you should use a modern "source port" for the best experience on modern hardware: Quake 3 Arena : id Software - Internet Archive
While the engine for is open-source, the full game itself (specifically the data files like pak0.pk3 ) is not officially free. To play it legally, you generally need to own the original game data. However, there are several ways to experience the game for free or through alternative open-source versions. 1. Free Alternatives & Demos
: This is a completely free and legal open-source game based on the Quake III engine. It features similar gameplay, weapons, and maps but uses entirely original assets.
: While it recently became a paid title on Steam, it remains the modernized version of the Quake III experience. 2. Playing with Modern Engines (Requires Game Files)
: You can legally download the official demo, which includes a limited number of maps and characters, from the Internet Archive .