The incident was reported around 1 PM, they said.
The workers were cleaning a sewer line in Lajpat Nagar near the Kabir Ram temple. They had gone inside the sewer and inhaled toxic fumes and were later found unconscious there, they said.
They were rushed to the AIIMS Trauma Centre where the doctors declared the "brought dead".
Two of the victims were identified as Joginder (32) and Annu (28), while the identity of the third was yet to ascertained, police said.