The collision happened on the A21 Bangor to Newtownards dual carriageway, near the Green Road junction, at 19:40 GMT on Thursday.
A number of people were taken to hospital after the crash. The road was closed overnight but has now reopened.
Insp David McClements appealed for anyone who witnessed the collision to contact police.
A man and a woman have been critically injured in a three-car crash in County Down.