The incident took place in a house in Housing Board Colony, Sector-3 Ballabgarh in Faridabad on last night.
Kuldip Datt, a medical representative by profession, was found lying unconscious on his bed while his wife and his son were found dead near him, with both of them showing signs of poisoning, said SHO Sandeep Singh More.
According to police, the incident came to light when Datt's nephew, who was passing through the house saw some shadow and made cries thinking some thieves have entered the house.
Datt was married to wife Saroj 11-year ago and they had 7-year-old son Krishna.
He was rushed to the nearby hospital. Police has found a knife from the bed.
A case has been registered and police is awaiting post mortem report, said More.