want to donate G3; how to purge data?

The best way would be to boot the system from the original system install CD (or any later version of Mac OS) and run Drive Setup or Disk Utility (the former for Mac OS 9, the latter in Mac OS X) and erase the drive. If you want to be completely certain that the data is gone, have the drive utility write zeros to the entire drive (I know this is an option in Mac OS X 10.2 or later; I don't remember for earlier versions of Mac OS X or Mac OS 9).

Initializing the drive with the Write Zeroes option will eliminate the data so well that only spies like the NSA (US National Security Agency) have any hope of getting part of it back, and only then if they take the drive apart and use special tools to read it.

8-way overwrite and other techniques are only needed if you have to meet Defense Department specifications for obliterating the nuclear secrets and other Classified documents you have hidden there.

If you are donating this Mac, please install some version of the Mc OS on it after you erase it -- most not-for-profits do not have the know-how to install the OS on a "blank" Hard Drive. They will likely run it exactly the way you give it to them.

Nice reference to the NSA, lol

Actually most grade schools have computer techs and usually only use Macs, don't know about colleges/Universities.