Deputy Superintendent of Police Ramashankar Rai said the accident took place near Dhamanpur village on NH-31 when a speeding truck rammed into the auto-rickshaw with eight occupants.
While three persons including the auto-rickshaw driver died on the spot, five others sustained injuries, Rai said.
The injured persons have been admitted at a government hospital.
The truck driver fled from the spot abandoning the vehicle which has been seized, the DSP said.