A fourth child has died following a suspected arson attack on a house.
Police and firefighters were called to the property in Barn Mead, Harlow, Essex, at about 1am on Monday.
A woman and three of her children were pronounced dead at the scene. A fourth child died later at the Princess Alexandra Hospital and a fifth remains in a critical condition. A man has been treated for minor injuries.
Neighbours raised the alarm after being woken in the early hours. They reported seeing flames and smoke billowing from the house.
Chief fire officer David Johnson said the blaze, which started downstairs while the family were asleep upstairs, had developed rapidly, suggesting some kind of fuel may have been used. He said the father was outside the house when the blaze started and is now helping police with their inquiries.
Senior investigating officer Detective Superintendent Rob Vinson said: "This is still the early stages of our investigation into the tragic deaths of five members of one family - a mother and four of her young children. At this time we are investigating to see whether this fire was started deliberately and need anyone with information to contact us."
The force confirmed that a car was also found alight a short distance from the house. The vehicle was not connected to the family, who are believed to be of Asian origin; however, the car fire is also being treated as arson. The house will be forensically examined to find out how the blaze started.
Mr Vinson added: "For the sake of the family, who are trying to come to terms with this tragic incident, I would ask for information from the public, especially in the Harlow area, to help us determine exactly what has happened."
Essex Fire and Rescue Service assistant divisional officer Steve Foster, incident commander, said: "Crews arriving at the scene did everything they could. They were faced with a rapidly developed fire and went straight into the burning building wearing breathing apparatus to bring people out as quickly as possible."
The children who died at the scene are believed to have been two boys, aged six and 13, and an 11-year-old girl.