What was found
LEVEL 1 AIR SYS 3 OFF ILLUMINATED FOURTEEN MINUTES INTO FLIGHT AT FL 280. ASSOCIATED BLEED (3) AND PACK (3) OFF. RED DUCT LINE ILLUMINATED ON AIR SYNOPTIC. COMPLIED WITH QRH. REF MM 36-00-01-5, PERFORMED DECAY CHECK OF #3 PNEUMATIC SYSTEM, DECAY CHECK PASSED AT 14 SECONDS FROM 20 PSI TO 15 PSI. MANIFOLD DETECTION CONTROL UNIT INDICATED NO WHITE BIT BALLS. PSC 3, MSC, AIU'S, VIA'S INDICATED NO CURRENT FAULTS SHOWED FOR CURRENT FLIGHT LEG. DUE TO HISTORY, IAW FIM 36-23-00-2-0. REMOVED AND REPLACED NR 3 AIU AS PRECAUTION. RTS PASSED ALONG WITH AIR SYSTEM TEST. NO FURTHER FAULTS FOUND. AIRCRAFT OK FOR SERVICE.