The incident occurred in the wee hours when the truck was heading towards Haryana, loaded with several bags of cement from a factory in Shakurpur, police said.
Passers-by informed police about the incident and the fire department was also called for rescue operation.
Six labourers were taken to the hospital, where three of them were declared brought dead, while the condition of the others are stated to be critical, police said.
During investigation it emerged that the truck driver, Kailash, who fled the spot, might have dozed off leading to the mishap, police said, adding a case has been registered hunt is on to nab the driver.