Quake III Arena and ioquake3 might not allow you to set widescreen resolutions for within the game.
The workarounds you will find on the internet are either editing the config or starting the game process with some config params.
Installing and using the app
- Extract the archive contents;
- Find the directory where Quake 3 is installed and move "Quake3ScreenResolution.exe" in there;
- Run "Quake3ScreenResolution.exe" and after choosing your prefered resolution and window mode, press "Create profile shortcut on Desktop".
Standard widescreen resolutions
16:9 aspect ratio 1024x576, 1152x648, 1280x720, 1600x900, 1920x1080 and 2560x1440
16:10 aspect ratio 1440x900, 1680x1050, 1920x1200, 2560x1600