Officer in-charge of Alamganj police station Rajesh Ranjan said the accident occured when driver of a fodder ladden tractor lost his control over the veicle and it overturned at Sursandy village.
It then rolled into the roadside ditch killing the three, one of whom is a woman and another a teenaged girl.
The injured were taken to Madhepura sadar hospital, where the condition of three of them was stated to be critical.