The trio went inside the well outside their house to clean it, but did not come out as they fell unconscious and three other villagers who moved in to rescue them were taken ill due to suffocating, police said.
Later villagers along with fire fighters pulled them out, but the man and his two sons were found dead.
The deceased were identified as Phainut Gomango (55) and his sons Presingh Gomango (24) and Atania Gomango (22), police said.