mistafeesh: If your G3 is one of the beige ones (without any factory USB ports), and you're running OS 9.x, you can install the necessary Apple USB support extensions by extracting them from the Installer Tome contained in the freely-downloadable OS 9.1 updater, using
TomeViewer. The ones you need are listed in this FAQ by Apple employee
Joerg Weise.
If your Mac has factory-installed USB 1.1 ports, all the USB software you need to support USB 1.1 operation of your adapter card should already be installed, and if the card doesn't work, it probably isn't backward-compatible with USB 1.1 (though most cards are) or isn't compatible with the version of OS 9 you're running.
You won't be able to realize USB 2.0 speed with your new card without upgrading to OS X 10.2.8 or higher.