PK3 is the extension given to the files most often used by Quake III. These files are used by the id Tech 3 game engine by id Software. It is a compressed format that uses ZIP compression. It contains all the resources used by the game, e.g. cardboard, audio data or maps. PK3 files can also be a base for creating modifications. They can be manually modified by the user.

About 20 game titles were created based on the id TECH 3 engine itself and the files it supports (including Medal of Honor, Call of Duty or Return to Castle Wolfenstein).