AS THE FLIGHT WAS CLIMBING THROUGH FL210, THE CREW HEARD AN EXPLOSION AND FLAMES WERE SEEN COMING FROM THE TAILPIPE OF NO.3 ENGINE. THE ENGINE WAS SECURED AND THE AIRPLANE LANDED UNEVENTFULLY. EXAMINATION REVEALED AN UNCONTAINED FAILURE. A PREEXISTING CRACK WAS FOUND IN THE NO.12 BLADE ON THE 4TH STAGE TURBINE WHEEEL. MAINTENANCE RECORDS DISCLOSED THE WHEEL HAD RECENTLY UNDERGONE A DYE PENETRANT INSPECTION. NO CRACKS WERE NOTED. THE ENGINE HAD ACCRUED 99 CYCLES AND 66 HOURS OVER A PERIOD OF 13 DAYS FROM THE TIME OF THE LAST INSPECTION AND THE TIME OF THE ENGINE FAILURE.