Assuming you have OSX installed (10.3 or 10.4 would be best), all you need to do is turn on file sharing from system prefs, specify which folders to share (remember to check permissions on each share too).
I would also set a static IP for the box to make it easier to troubleshoot and connect to.
Once it's setup and on the network, your other macs can connect to it via the finder window (depending on OS version) or, more efficiently via the connect to server dialogue box under finder->go menu (Command+K is the shortcut).
From there, just enter the DNS name you assigned (eg //powermacg3.local) or IP (eg //10.10.0.23). Press the + button to add to the list of machines for the future use. Just log in with the G3's credentials and select the share to mount.
Very easy to do. If you're running OS9, it's a different story.