Two 18-year-old women and a 19-year-old man were injured during the early morning accident in Hardgate, near Clydebank.
The car they were travelling in crashed in Glasgow Road around 4am today.
The road was closed for eight hours while investigations were carried out.
Three ambulances and a paramedic response unit were sent to the scene, near Clydebank and District Golf Club.
It is understood the car left the road and careered into a grass embankment in a wooded area. It is then thought to have flipped on to its side.
The 19-year-old man was rushed to Glasgow's Southern General Hospital suffering from serious head injuries.
One of the 18-year-old's was taken to the Glasgow Royal Infimary suffering from serious leg injuries while the second was being treated at the Western Infirmary for minor leg injuries.
Investigations are continuing and the road was closed in both directions for several hours, including during rush hour. It reopened around 11am
Meanwhile, a woman was rushed to hospital today after being hit by a bus in Glasgow city centre.
She was crossing Killermont Street, near Buchanan Bus Station, at around 9.45am, when she was knocked down.
Paramedics treated the woman at the scene, before she was taken by ambulance to hospital.
Killermont Street was closed between West Nile Street and North Hanover Street, while a police investigation got under way.
A police spokeswoman said: "A female pedestrian has been injured, however, her condition is not known."