It happened on the Acle-bound carriageway near Halvergate at about 12.20pm and involved a silver SEAT Leon and a silver Renault Clio.
The female driver of the Renault Clio received serious arm and internal injuries and was taken to the Norfolk and Norwich University Hospital, where she remains in a stable condition.
A woman, who was a passenger in the Renault Clio was taken to the James Paget Hospital with serious injuries and remains in a stable condition. A child in the same car was taken to the James Paget Hospital with a serious arm injury, and she remains in hospital. In addition, a young boy travelling in the Renault Clio was taken to the Norfolk and Norwich University Hospital with a serious head injury. He remains in hospital in a stable condition.
Officers are keen to hear from anyone who may have witnessed the collision, has dash camera footage or information concerning the manner of driving of the vehicles before the collision.
Anyone with information or dash camera footage should contact PC Michael Stolworthy of Acle Roads & Armed Policing Team on 101 or email: Michael.firstname.lastname@example.org quoting incident number 145 of 9 March 2020.