The incident occurred when bombs were being made in the house of one Giasu Seikh in the area bordering Bangladesh at about 1 AM, Superintendent of Police Syed Waqar Raza said.
Three persons died on the spot and four others were seriously injured, he said, adding that "local rowdies" were involved in the incident. Giasu was absconding.
Polling was held in the district on April 17.