What was found
(CAN) A/C WAS SITTING AT THE GATE WITH AC EXTERNAL POWER SELECTED ON AND HYDRAULIC SYSTEM NR 3 PRESSURIZED. AC POWER HAS BEEN SELECTED OFF WITH HYDRAULIC NR 3 SYSTEM STILL PRESSURIZED WHICH CAUSED THE NLG TO COLLAPSE. DUE TO CHOCKS BEING INSTALLED, NLG DID NOT COLLAPSE COMPLETELY AND NO DAMAGE OCCURRED TO THE A/C. NO PASSENGERS WERE ONBOARD AND NO PERSONAL WERE INJURED. NOSE LANDING GEAR SELECTOR VALVE REPLACED, GEAR SWINGS PERFORMED AND LANDING GEAR OPS CHECKED SERVICEABLE IN ACCORDANCE WITH THE AMM.