I don't know about the beige G3's, but early B&W G3's could not accept a slave drive. The problem was with the motherboard, and was fixed in the second revision. You might want to check Apple's web site for the answers you seek.
Assuming that you can add a second hard drive as a slave, make sure you have the jumpers set correctly. For info on how to set the jumpers, see the web site for the HD manufacturer.