A three-year-old boy, a woman aged in her 30s and a man in his 40s have been found dead at an address in Brentford, west London.
The Metropolitan Police said officers were not looking for anyone else in connection with the incident.
Officers forced entry to the property in Clayponds Lane, Brentford at about 12.50am on Tuesday, following concerns for the welfare of people inside.
The male was suffering from stab injuries. London Ambulance Service attended but he was pronounced dead at the scene.
It is thought all three people were known to each other.
Anyone who has information that could assist police with their investigation are asked to call 101 ref CAD 7365/6 Oct.