The incident occurred this morning when the labourers were cleaning the well, Raigarh Additional Superintendent of Police U B S Chauhan said.
The victims were identified as Daul Kumar Nishad (24), Krishna Kumar Rathiya (18) and Purushottam Rathiya (20), he said.
The trio went inside the well, which had less water, at the house of Hariram Rathiya (35) in Darripara locality, to clean it, but fell unconscious after inhaling the gas that had accumulated inside it, Chauhan said, adding they died on the spot.
The bodies have been sent to Tamnar hospital for postmortem, the ASP said, adding a case has been registered in this connection.
Investigations are underway in the matter, the ASP added.