The body was discovered on Wednesday evening after the car owner called the police, said Lt-Col Moatshe Ngoepe.
“The three had locked themselves in the car while playing for hours and they were found by the owner when he got back from work.”
Ngoepe said the owner noticed that the car lights were on and he called the police.
Two of the children were unharmed.
“We are not sure what killed the other one. The post moterm will determine the cause of death.”
An inquest case was opened and police were investigating.