The incident took place in a house belonging to Pawan Sharma (60) as a labourer Rameshwar Raman (41) entered into a toilet tank to carry out repair works.
But after he did not come out nor responded to their call, Sharma and the contractor Gauri Shankar Rai (30) too went inside, but all three of died due to asphyxiation, sources said.
The bodies of three men have been fished out and sent for post-mortem, they said.