The incident took place near Chapra High School when a car carrying four members of family rammed into a speeding truck coming from opposite direction, Meenapur police station in-charge Mithilesh Jha said.
While three persons, identified as Moti-ur Rahman (61), his wife Nasima Khatun (55) and grandson Md. Adnan (12), died on the spot, Rahman's son Imran (20) sustained injuries, he said.
The bodies have been sent for post-mortem at Sri Krishna Medical College and Hospital (SKMCH).
The injured person was also brought at the same hospital for treatment.