Internal Power Supply on G3

I assume that you're referring to the beige mini-tower and not the desktop model. Although the power supplies have the same output rating and the main connectors have the same pin-outs, they have different part numbers. A B&W G3's power supply is part #661-2107, while the beige G3's is part #661-1496. Which power supply do you suspect needs replacing and have you tested it with a voltmeter? Different symptoms can give the impression that the power supply is dead, when it isn't. If needed, you can replace the power supply in a B&W G3 with an ATX (PC) supply, provided that you make one alteration to the wiring harness. Cut off the -5 volt lead at pin 18 and connect a ground to it instead. Using a power supply extension cable for the modification is a better method, than permanently altering the actual PS.

When replacing the PSU with an ATX PSU take the G3 PSU with you to make sure the AC plug location matches so it will fit correctly in the G3 case. Some ATX PSU's are a little different and don't always line up correctly with the rear of the Mac case.

And the PSU extension cable hack Jeff mentioned it the way to go since some ATX PSU cables may not be long enough to connect to the Mac logic board. I put an ATX PSU in a Sawtooth G4 and the cable could use another 6 inches of length to route the cables correctly, but it works. There are plenty of used ATX PSU's cheap or even free.