Gate voltage and drain current relation
If you put 3V on the gate to source then the drain current will be zero until you apply a drain-source voltage (usually via current limiting device such as a resistor). 

It doesn't convert input voltage to output current like a solar panel converts light to voltage. It doesn't convert energy in one form to energy of another (in case you thought it did).

Here's a typical graph: -



When 3V is on the gate (relative to source) and at Vds of zero volts the drain current is also zero but, if you increase the drain voltage the current rises from zero to a value dependent on the graph. Note that each MOSFET will have their own particular graph in their data sheet.