What was found
(CAN) DURING TAXI , CREW WAS ADVISED OF FIRE AND SMOKE ON RT SIDE OF THE AIRCRAFT, NR 3 ENGINE SHUT DOWN , BUT REALIZED THAT THE FIRE WAS COMING FROM THE APU TAIL PIPE. APU SHUT DOWN. CREW REPORTED THAT THEY HAD NO PROBLEM WHEN THEY STARTED THE APU FEW MINUTES BEFORE. INVESTIGATION CARRIED OUT, OIL CONTENT ALMOST EMPTY AND HEAVY BLUE SMOKE WAS COMING OUT OF THE EXHAUST. APU REPLACED AND AIRCRAFT RETURNED TO SERVICE. WAITING FOR SHOP REPORT.