THE NO. 3 ENG TORCHED DURING A START ATTEMPT AND RESULTED IN A INTERNAL ENG TAILCONE FIRE. SEVERAL PAXS PANICKED UPON SEEING THE TORCH FLAME/TAILCONE FIRE AND BETWEEN 30 TO 50 PAXS MADE AN UNAUTHORIZED EVACUATION OF THE ACFT VIA THE L3 AND L4 EXITS. PAXS INITIATED THE EVACUATION AT THE UNATTENDED L3 EXIT. A F/A MISTAKENLY ASSUMED THAT AN AUTHORIZED EVACUATION WAS IN EFFECT AND THEREFORE OPENED L4 AND ALLOWED FURTHER PAXS TO EVACUATE. THE EVACUATION OF THE ACFT WAS LATER STOPPED AND BROUGHT UNDER CONTROL. THE FLT CREW WAS AT FIRST NEITHER AWARE OF THE ENG TORCH AND SUBSEQUENT TAILCONE FIRE NOR THE EVACUATION. WHEN THEY LEARNED OF THE TAILCONE FIRE THEY MOTORED THE ENG IN ORDER TO EXTINGUISH THE FIRE WHICH THEY BELIEVED WAS UNDER CONTROL AND THEREFORE REQUESTED NO EMERGENCY ASSISTANCE. HOWEVER, ATC WAS ALERTED BY PILOTS IN NEARBY ACFT ABOUT THE OCCURRENCE AND ATC NOTIFIED CFR WHO RESPONDED TO THE SITE. THE NOS. 1 & 2 ENGS REMAINED RUNNING DURING THE INCIDENT AND CREATED A HAZARDOUS CONDITION FOR CFR/EVACUEES. ENGS WERE LATER STOPPED.