A FOUR-year-old boy, his brother, 9, and their sister, 11, were airlifted to hospital in Melbourne last night after the car they were travelling in hit a tree, leaving them with life-threatening injuries.Police believe the accident occurred just after 5pm, when a black Toyota wagon travelling east on the Glenelg Highway at Pittong veered off the road and went down an embankment.The driver of the vehicle, a 38-year-old woman from Hamilton, suffered abdominal, arm and head injuries in the accident.She was taken by a fixed-wing aircraft to the Royal Melbourne Hospital in a serious but stable condition.Her three children were taken in separate air ambulances to the Royal Children’s Hospital.A spokesman for Ambulance Victoria said the four-year-old boy sustained head injuries in the accident, while the 11-year-old girl suffered injuries to her legs, pelvis and arms.Both children were placed in induced comas at the scene and airlifted to hospital in a critical condition.The ambulance spokesman said the nine-year-old boy was the last to be freed, and suffered significant leg injuries because he had been trapped for so long.Last night he was in a serious but stable condition in hospital.Ballarat Highway Patrol Senior Constable Craig Walker said emergency crews worked for more than an hour-and-a-half to free the children, and spent more than three hours at the scene.Senior Constable Walker said further inquiries were needed to determine the cause of the accident.He said all four occupants in the vehicle were wearing seatbelts.Anyone with information should call Crime Stoppers on1800 333 000.Follow @jennifergreive