Try GTK Radiant 1.6.5 icculus.org/gtkradiant/.
I sort of struggled to learn how to use it, but a lot of stuff I needed to know that wasn't in the manual I looked up here:ws.q3df.org/level_design/.
You could also try older versions of GTKRadiant robotrenegade...nt/downloads.html
if you want, but I use 1.6.6.
With GTKRadiant you not only can make maps for Quake 3, but also games like RTCW and Jedi Academy.