What was found
(CAN) NR 3 ENGINE OIL QUANTITY WENT TO ZERO IN-FLIGHT. OIL PRESSURE DROPPED TO 38 PSI, LOW OIL PRESSURE LIGHT ILLUMINATED AND THE ENGINE WAS SHUT DOWN. THE ENGINE MAINTAINED A POSITIVE OIL PRESSURE ABOVE 20 PSI FOR THE DURATION OF THE FLIGHT. POST-FLIGHT TROUBLESHOOTING FOUND A PLUG, ON THE ENGINE GEARBOX, FORWARD OF THE HYDRAULIC PUMP MOUNTING PAD LEAKING. A NEW HELICOIL WAS INSTALLED. THE PLUG WAS REINSTALLED AND A LEAK CHECK WAS CARRIED OUT SERVICEABLE. AIRCRAFT RETURNED TO SERVICE.