Emergency services were called to the B2112 Common Lane at Ditchling Common at 2.30am after a car left the road and collided with a tree before overturning near the Oaks Poultry Farm, said police.
A man and two women, all in their 20s and from the Haywards Heath area, sustained serious injuries and had to be freed from the vehicle, a blue hatchback, by firefighters.
All were taken by ambulance to the Royal Sussex County Hospital, Brighton, said police.
The road was closed until 6.30am, added police.
Anyone who saw what happened or noticed the vehicle being driven shortly beforehand, who may have dash-cam footage or who has other relevant information, is asked to report details online at www.sussex.police.uk/contact/af/contact-us.
Alternatively, people can email [email protected] or call 101, quoting Operation Alto.