To play the Windows version of Quake3 in OS X, download and install the 1.32 OS X update from id Software's Quake3 Gold page (click on the "updates" link at the bottom of the page). The installer will create a Quake3 folder in your Applications folder. Copy the "pak0.pk3" file from the /Quake3/baseq3/ folder on the Quake3 CD to the "baseq3" folder in the newly created Quake3 folder. Run Quake3 and enter your CD key and you're all set.
[robg adds: Since id distributes a hybrid PC/Mac CD-ROM, I don't believe they'll have an issue with a single user purchasing one license, but if I'm wrong, I'm sure I'll hear about it.]

