What was found
ATL - FLT 1432 - THE NR 3 ENGINE FIRE LIGHT AND NR 3 START VALVE LIGHT BOTH ILLUMINATED INTERMITTENTLY IN-FLIGHT WITH A LOSS OF CABIN PRESSURE. ACFT RETURNED TO ATL WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND NR 3 ENGINE'S 13TH STAGE PNEUMATIC DUCT BLOWN. NO OTHER DAMAGE WAS NOTED. THE 13TH STAGE DUCT REMOVED AND REPLACED. IN ADDITION, NR 1 ENGINE PRE-COOLER HEAT EXCHANGER FOUND INOPERATIVE. PRE-COOLER HEAT EXCHANGER REMOVED AND REPLACED. ACFT PRESSURIZED AND A DECAY CHECK PERFORMED WITH ALL SYSTEMS CHECKING NORMAL. (X)