using atx power supply with motors

I want to use a ATX power supply (from a computer) to supply power to 3 motors 12 volt and 3 volt, how do i connect the different electric wires (which ground etc.)?

Does anybody have a sketch or drawing about this?

Keep all the grounds common to the supply and arduino.

Then decide how you want to power the motors, see:-

Keep the + of the 12V to power the higher voltage motors and the 3V for the lower powered ones. You can even use the 5V to power the Arduino. However be aware that the power supply can give out a lot of current so it is rather unforgiving about shorts.