The cockpit crew was in the process of starting the Auxiliary Power Unit (APU) when passengers saw flames coming from the right engine. A passenger was heard to scream ‘Fire’ at which time the uncommanded evacuation was initiated by the passengers. The captain attempted to calm the passengers with a PA announcement. Passengers exited the airplane via the over wing exit, the rear airstair door, and the main entry door onto the jet bridge. A ten year old boy received a broken arm and two other passengers received minor leg and ankle injuries as they jumped off the wing. Probable Cause: the passenger initiated evacuation of the airplane. A factor was the auxiliary power unit fire.