Verified answer

http://ioquake3.org/

Id Tech 3

For the best answers, search on this site https://shorturl.im/aw1ae

Check out "Code3Arena" (website) or "Focus on Mod Programming in Quake 3 Arena" (book). Both of which are basically identical but offer TONSSSS of information regarding where/how to start hacking away. :) Have fun bro