I previously bought a cheap ps3 compatible wireless controller, only to find out later it didnt work on the retroarch. Later I successfully paired the retroarch with an official sony ps3 controller. Now Im looking to get another ps3 wireless controller, so my question is, for example, could a cheap Chinese knock-off of ps3 controllers with 6 axis dual shock be compatible to be paired with the retroarch on my ipad 3? The cheap ps3 controllers I speak of are found on ebay ranging 10 to 20 dollars, made in china.
Yes the Official Dual Shock 3 controller will work 
No, I have tried using a cheap, non-official Dual Shock 3 controller and it failed to pair. RetroArch is only compatible with the expensive, official Sony-made controllers.