The official Quake 3 Arena site has the Quake III Arena test release for G3 Macs for download. There was a brief false-start earlier when a version was briefly posted and pulled, but that's been cleared up. Here's a local copy, with mirrors of the 21.4 MB file, and here's the word on the release:

Before Downloading, READ THIS:

This is the Mac version of the test
Linux and Win32 version will be released as soon as possible

Q3TEST is for TESTING new tech and multiplayer code.
Q3TEST is NOT the game "Quake 3 Arena".
Q3TEST is ADDICTIVE. Be sure to get some sleep.

Known Mac Issues

• Sometimes the Mac build crashes in low memory conditions, we're looking for help in tracking this.
• The sound latency can be improved.
• Currently the game will only run on the main screen in a multi-monitor setup.
• The overbright bits we use to change the way lightmaps work in the PC build is not currently in the Mac build.

We are running servers at;

• 192.246.40.37:27960 -- q3test1
• 192.246.40.37:27961 -- q3test1
• 192.246.40.37:27962 -- q3test1
• 192.246.40.37:27963 -- q3test2