The deceased were identified as Munnilal Mishra (35), Akhileshwar (25) and Muneshwar (20), Sohagi police station in-charge Ayachi Dube said.
The three had climbed down into the well to repair a faulty water pump, but fell unconscious after inhaling the gas that had collected inside.
They were taken to a government hospital at Tyonthar where they died, he said.