This is easy 😉
In your Applications folder, create a new folder called "Quake III Arena". Place the Universal Binary application inside. In the new folder, create yourself a folder called "baseq3".
From your PC install, navigate to the "baseq3" folder. Copy over all the files with names such as pakX.pk3 (where X is 0-8 or so) and place them in the Macs "baseq3" folder.
When this is done, that's it and it should run. This is a barebones install that you can use as a base. To install mods/maps/configs keep the file/folder structure the same as the PC version and they will all run. I'll post up a screenshot if you wish.