Rating
Nobody has played it on Walnut yet, so it has no medal.
Installation
In Walnut Games, the desktop app: Install, then Add game files... and pick your copy of Quake III Arena: the disc in the drive, an ISO, BIN/CUE or MDF image, an installer or a folder. The game data is not part of the package, and Walnut does not offer it.
What works: Quake III Arena's baseq3/pak0.pk3: its CD or an ISO of it, an installed copy (Steam, GOG) or GOG's Quake III Gold installer (with Team Arena).
Optional, from a copy that has them: the expansion: Also add in the app.
More ways (a terminal, game-data-packager on any Debian system, copying the files by hand): Guides.
Or from a terminal, from Walnut's apt repository:
sudo apt install ioquake3
Known issues
- GOG's installer is unpacked whole: importing from it needs about 2 GB free for Quake III Arena (its folder or the CD need less, about 1.6 GB).
- Quit from the game's menu: stopping it from outside can leave it hung on its last frame.
The game's home page: https://ioquake3.org/. License: GPL-2.0-or-later engine, commercial game data.