For my own personal reference I am putting some useful links about mapping for Quake 3. All of these websites were of tremendous help while learning about mapping.
Quake 3 World Level Editing forum : The best forum for Quake 3 mappers, enough said.
Robot Renegades : Everything you need to know about GtkRadiant, Shaders and Q3Map2.
Using hi-resolution external lightmap : The title says it, if you don’t know about external lightmap for Quake 3, check this out.
Quake 3 Level design @ Worldspawn : A collection of documents/tutorials for mapping with Quake 3.