24V 3A from ATX power supply?
You can open the supply and look for the TL431 chip.  There will be two resistors forming a voltage divider from the 5V rail to the 2.5V that the TL431 compares against.   If you calculate the right resistor values, you can set this divider to output half the voltage it currently does.  Then you can get the output up so that the +5V rail becomes +10V and the +12V rail becomes +24V.  You will probably need to install new output filter capacitors of higher voltage rating as well.  And if you modify an ATX supply this way, please cut off the ATX connector and install something else, so it won't plug in to a motherboard.