id Tech 3 is a game engine developed by id Software for Quake III Arena and has been used in many games under the Quake III Arena engine and Quake III: Team Arena engine branding. During its time, it competed with the Unreal engine; both engines were widely licensed.
id Tech 3 is not based on id Tech 2; it was written from scratch. Successor id Tech 4 was derived from id Tech 3.
At QuakeCon 2005, John Carmack announced that the Quake III source code would be released under the GNU General Public License (version 2), and it was released on August 19, 2005. The code can be downloaded from id's FTP site.
Read more about Id Tech 3: Ioquake3