A PS3 pad requires 3rd party software to work, at least it did last time I looked, it will detect and install basic HID drivers if plugged in via USB but the controller can't be used in that state.
http://www.epforums.org/showthread.php?77890 has two options for this, personally I could not get either working well enough for me but I have a PS2 to USB controller converter that does work perfect, if you have a PSX or PS2 pad this is a very cheap option the converters are very cheap now, sometimes called a PS2 to PS3 pad converter but they work on PC fine and still convert to USB or a PSX/PS2 to USB pad converter
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