The incident happened at Samanpur village under the jurisdiction of Kundwa Chainpur police station.
The deceased were identified as Naresh Rai (50), his grandson Prince Kumar (05) and granddaughter Menaka Kumari (12), officer-in-charge of Kundwa Chainpur police station, Abhay Kumar said.
Rai had kept some pesticide in his house and Menaka, said to be mentally retarded, inadvertently mixed some pesticide with tea leaves, milk and sugar while preparing tea this morning, Kumar said.
After consuming the tea, the trio went to work in the nearby fields where they all fell sick and died while being taken to a hospital, he said.