The incident occurred when the three had gone inside the sewer to clean it, police said.
The labourers, who belonged to Panchkula, are suspected to have inhaled some poisonous gas inside the sewer, police said.
The authorities were alerted by a fourth labourer who was also present at the spot.
"We are conducting investigations into the case," Assistant Superintendent of Police (South), Navdeep Singh Brar said.
Asked if the labourers were equipped with safety equipment to carry out the cleaning operation, Brar said all these things are being investigated.