What was found
THE FIRE WARNING BELL SOUNDED FOLLOWED BY ILLUMINATION OF THE NR 3 ENGINE FIRE WARNING LIGHT ON CLIMB-OUT OF IAH AT ABOUT 900 FEET. THE NR 3 ENGINE POWER WAS REDUCED AND THE FIRE WARNINGS CEASED. A VALID FIRE TEST WAS RUN. THE LIGHT AND BELL CAME ON AGAIN WHEN ENGINE WAS RETURNED TO CLIMB POWER. FUEL WAS DUMPED AND THE AIRCRAFT WAS RETURNED TO IAH FOR A NORMAL LANDING WITH NR 3 ENGINE AT REDUCED POWER. MAINTENANCE FOUND THE NR 3 ENGINE ANTI-ICE DUCT BLOWN. THE DUCT WAS REMOVED AND REPLACED. THE NR 3 ENGINE WAS RUN WITH NO LEAKS NOTED AND OPERATION CHECKED GOOD.