Two adults and a child died in a five-vehicle crash after a car stopped at the side of the road, police said.
Officers have described the chain of events that led to the pile-up on the A38 in Cornwall on Sunday.
They said a 53-year-old Plymouth man died after getting out of the stopped car and being hit by a vehicle travelling in the opposite direction.
A 32-year-old Saltash woman and her six-year-old son in another car also died.
Her four-year-old son has “potentially serious head injuries” but is said to be improving at Bristol Children’s Hospital.
Ch Insp Adrian Leisk from Devon and Cornwall Police said the families involved are “devastated”.
One officer at the scene tweeted that it was the “most heartbreaking shift” in her six-year police career.
The pile-up happened between Tideford and Landrake on the busy main road at about 11:15 BST.
Ch Insp Leisk said a vehicle pulled over and stopped on the single-lane section of the Plymouth-bound carriageway.
“Someone has got out of that vehicle and has been struck by a vehicle travelling in the opposite direction,” he said.
“Then a number of other vehicles have become involved and sadly two occupants of another vehicle have died.
“Every road death is a tragedy but it brings it into sharper focus and really hits home when there are children involved.”
The driver of the car that stopped was “in a state of shock” and being questioned by police, he added.
The road, which was closed for most of Sunday, has reopened.
Six further people were taken to Derriford Hospital in Plymouth with minor injuries. Two others were treated at the scene by paramedics.