freechragecontroller.org. Current is flowing even though M3's gate is not powered?
The circuit in the schematic is a power section of a Buck converter.  Read-up on buck converters, because that's the background information that you might be missing.  

M3 provides synchronous rectification, which is an improvement on the basic buck converter in efficiency and heat dissipation.  As M3 is turned on and off, the current through M3 would be the same as if it were a diode.  However, the turned on MOSFET doesn't have a forward voltage drop of a diode.