A car traveling on the B3081 Verwood Road between Verwood and Ringwood ended up on its roof after a serious collision in the early hours of Sunday morning.
The incident happened around 2am near to the Bakers Hanging junction to Alderholt.
A silver Volkswagen Polo had left the road and landed on its roof.
A 21-year-old woman from Verwood sustained serious face and head injuries and was taken to Southampton General Hospital for treatment. Her condition is described as very serious but stable. Her family has been informed and are with her.
An 18-year-old woman from Verwood, 26-year-old man from West Moors and a 44-year-old man from Verwood have been arrested on suspicion of driving offences and are assisting officers with their enquiries.
The road was closed between the Ebblake Industrial Estate and the A31 junction while emergency services treated the injured woman and for an examination of the scene to take place. It reopened at 6.50am Sunday morning.
Police Sergeant Craig Tatton, of the traffic unit, said: “An investigation is underway to establish the circumstances of this collision and I am appealing to any motorists who were travelling in this area at the relevant time and either saw the Polo being driving immediately prior to the incident or witnessed the collision to please contact Dorset Police.
“I am particularly keen to hear from any motorists who captured anything of relevance on dashcam.”
Anyone with information is asked to contact Dorset Police by calling 101 quoting incident number 18:58.