The doom3linux1.1.1282demo.x86.run file you have requested: Official Releases / Demos / DOOM 3 Demo for Linux - 462.61mb is not available. Perhaps try browsing all files for the game Doom III and upload any files you want to share or are missing. This is the only information we could locate:
Singleplayer Linux demo for DOOM 3. Includes the Mars City mission (three levels).