There are lots of places to add extra drives in a Desktop G3:

-Where the Zip drive is and above that (make a special bracket)

-Where the floppy drive is, another Optical drive can be placed there if the front of the case is modified. You'd switch to a USB floppy since the internal floppy isn't supported by Mac OS X.

-Along the floor, where the internal drive is located, you could get a smaller drive and place two in there.

The problem is that the power supply can't handle all those drives. You could use a PC power supply: The conversion is reportedly easy and also involves moving a jumper on the G3 motherboard.