Many thanks for the advice 
The game works great - I installed normal Quake and the 1.09 patch in SheepShaver on my PC, so as to avoid all the old stuff from screwing up my G3's installation, stuffed up the folder and copied it into OS X, put GLQuake for OS X in that folder and it works! 
These must have been such amazing machines in their day, Quake was the first game I had on my first PC that I got new in '98 when this G3 was also new, and it runs better on the G3 than it did on there (I only had the normal non-accelerated version then for a start, as I had some rubbish 2 MB non-3D card), even under OS X when you would expect it to run faster under Windows 95 as it's a much lighter OS.
The game works great - I installed normal Quake and the 1.09 patch in SheepShaver on my PC, so as to avoid all the old stuff from screwing up my G3's installation, stuffed up the folder and copied it into OS X, put GLQuake for OS X in that folder and it works! These must have been such amazing machines in their day, Quake was the first game I had on my first PC that I got new in '98 when this G3 was also new, and it runs better on the G3 than it did on there (I only had the normal non-accelerated version then for a start, as I had some rubbish 2 MB non-3D card), even under OS X when you would expect it to run faster under Windows 95 as it's a much lighter OS.