Hi, had a look around the net, A3000 PS are rare to get hold of, AmigaManiac has a converter cable for an ATX to a A3000 supply.
If you get the dimensions of the A3000 power supply and the wattage you can replace it with a new modern power supply & add the cable to it for use on your A3000s.
AmigaManiac link
http://www.amigamaniac.com/atx_to_amiga.html Good luck & I hope this helps :laugh1:
