What was found
WHILE DOING DELAYED ENGINE START ON ENG 2 (2 START ALL NORMAL) NOTICED ENGINE 3 IDLING HIGHER THAN ENGINE 1. ALL OTHER INDICATIONS ON ENG 3 NORMAL (EGT,FF,ETC.). DECIDED TO SHUT NR 3 DOWN AND RESTART SHUT DOWN NR 3, WAITED UNTIL ALL INDICATIONS WERE ZERO +30 SECONDS. ATTEMPTED RESTART ON NR 3. ALL INDICATIONS NORMAL ON INITIAL START-SHORTLY AFTER F/O RAISED THE FUEL LEVER AT 20% N2, AN AIRCRAFT OVER GROUND CONTROL REPORTED SMOKE/FLAMES COMING OUT OF NR 3 ENGINE. COMPLIED WITH TAILPIPE FIRE PROCEDURES, SHUT OFF FUEL LEVER, MOTORED ENGINE. AIRCRAFT BEHIND US STILL REPORTING SMOKE, FIRED BOTH FIRE BOTTLES. REPLACED NR 3 ENGINE.