Typically its the CPU_FAN header where you'd normally plug in your CPU cooler. Is there any documentation that specifies what to use?
No there isn't
From Corsair's blog " it is not recommended to have the pump on a controller fan output. For power to the pump, be sure to use a header that supplies a continuous and uninterrupted 12v."
My Mobo manual says nothing about the 3 pin connectors,