Police said the workers were cleaning a sewer line near the Kabir Ram temple when the incident took place. They went inside the sewer and inhaled toxic fumes and were later found unconscious there. The incident was reported around 1 PM.
They were rushed to the AIIMS Trauma Centre where the doctors declared them "brought dead", they said.
BJP's Delhi unit chief Manoj Tiwari slammed the Delhi government over the deaths and asked Lieutenant Governor Anil Baijal for a probe into the incident.
"This is not the first incident of death of safai karamcharis (sanitation worker) during sewer cleaning and the attitude of Delhi Jal Board officials as well as the Arvind Kejriwal government continues to be callous," Tiwari said.
Two of the victims were identified as Joginder (32) and Annu (28), while the identity of the third was yet to ascertained, police said.