NO. 3 ENG DEVELOPED AN EXHAUST STACK FIRE DURING THE SECOND ATTEMPTED START FOLLOWING TAXI OUT. THE 1ST START WAS ABORTED WHEN CREW BECAME AWARE OF A 15 MINUTE DELAY BEFORE TAKEOFF. DURING THE 2ND START, EXCESS FUEL TORCHED IN THE COMBUSTORS AND THE EXHAUST STACK. NO EVACUATION WAS ORDERED BY RESPONSIBILE CREW MEMBERS, BUT PANICKED PASSENGERS FORCED A FLT ATTENDANT TO OPEN THE LEFT REAR EXIT AND THE SLIDE DEPLOYED. 15 PASSENGERS EVACED BEFORE ORDER COULD BE RESTORED. ONE PASSENGER RECEIVED MINOR INJURIES WHEN A SECOND PASSENGER ROLLED OVER THE TOP OF HER AT THE BOTTOM OF THE SLIDE. FIRE WAS EXTINGUISED WITHIN 2 MINUTES BY CFR CREWS. INSP OF THE ACFT DID NOT REVEAL ANY DAMAGE AND INSP OF ENG REVEALED NO DEFICIENCIES. SUSPECT EXCESSIVE START FUEL INTRODUCED INTO CUMBUSTORS DURING FIRST ABORTED START OR SECOND START.