What was found
AT 118 INTO FLIGHT AT FL 320 GOT LVL 1 "HYD 3 ELEV OFF" ALERT. FLUID LEVEL IN NR 3 WAS 4.3 GALS AND STARTED DECREASING AT A RATE OF .1 GAL EVERY 3 MINUTES. WHEN 3.5 WAS REACHED;, TURNED HYD SYSTEM TO MANUAL AND TURNED OFF HYD 3 LT AND RT PUMPS AND ALSO THE 1-3 AND 2-3 RMPS. FLUID LEVEL STABILIZED AND CLIMBED BACK TO 3.9 GAL. GEAR WAS LOWER BY ALTERNATE MEANS. NR 3 HYD WAS REPRESSURIZED AND QTY WENT TO 1.2, HYD NR 3 WAS DEPRESSURIZED, UNEVENTFUL LANDING AND TAXI IN, TOWED INTOP THE GATE. R & R BAD UNION, PACKING AND O-RING ON UPPER HORIZONTAL STAB PRIMARY-TRIM CONTROL-VALVE REF AMM 27-43-40-4. LEAK AND OPS CHECK GOOD.