What was found
(CAN) ENROUTE, HAD AN ENGINE FIRE WARNING. ENG NR 3 DRILL AND CHECK LIST COMPLETED AND NR 1 BOTTLE DISCHARGED. TROUBLESHOOTING CARRIED OUT AND COULD NOT FAULT ON GROUND. WITH ENGINE RUNNING, CARRIED OUT A LEAK CHECK OF PNEUMATIC DUCTS ON THE ENG AND AROUND THE FIREWALL. FOUND 1 DUCT CLAMP LEAKING AT FIREWALL, NEAR THE FIRE LOOP. TRIED TO TIGHTEN CLAMP BUT WOULD NOT STOP THE LEAK. REMOVED AND REPLACED DUCT CLAMP AND LEAK CHECKED SERVICEABLE. ALSO INSPECTED WIRING ON NR 3 ENG, NO FAULTS FOUND. CLOSED NR 3 ENG COWLS AND CARRIED OUT ENGINE RUN TO 1.6 EPR, NO FAULT NOTED. LT ENGINE FIRE BOTTLE REPLACED.