If you hold down the "alt" key (may also be called "option") immediately after you hear the startup tone this will allow you to choose your startup disk. Bootable CD's will also show as available startup options.
If you know you want to boot directly from a CD then hold the "C" key down immediately after you hear the startup tone.
If you want to boot in target disc mode then hold the "T" key down immediately after you hear the startup tone.
**Remember this is assuming the commands are the same when working with G3's. (I have used them working with G4's & G5's)
Hope this helps!