38-year-old Dilip Ram, who went to his in-laws house at Bharojo village, did not return to his home at Baheri village and instead went into a forest where he allegedly prepared a poison potion. He allegedly forced his wife Kiran Devi and 7-year-old son Santanu to drink this, police said.
All of them fell unconscious, police said, adding that a little while later, Ram regained consciousness and informed his father of the incident over phone.
The relatives reached the spot and rushed them to a hospital, where doctors declared the woman and the boy brought dead.
A critically ill Ram was taken to Hazaribagh Sadar hospital where he too died.
The police were investigating reports that Ram had allegedly taken the drastic step as he was in debt of lakhs of rupees.