At least 10 people have died in similar incidents in the national capital region over the last two months.
SP (City) Arun Kumar Singh said the three men got stuck in the narrow sewer line yesterday and choked to death on toxic gases.
They have been identified as Rajesh, Vikas and Ravinder.
The bodies were taken out late last night.
The police said Sohan Lal, the private contractor who had hired the three workers to clean the sewer line, is on the run.
The police said an FIR will be registered against the contractor if a complaint is filed in this regard.