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.

The visitors took the lead three times in the first half thanks to tries from Justin Horo, Luke Walsh and Tony Gigot.
However, scores from Theo Fages, Alex Walmsley and Louie McCarthy-Scarbrook gave Saints the lead at the interval.
Horo's second try put the Dragons back in front but tries from Jonny Lomax, Mark Percival, Luke Thompson and Ryan Morgan guided the hosts to victory.
Luke Walsh's goal after Richie Myler's try moved the French side to back within two points before the Saints crossed three times in the final 10 minutes.
A third win in four matches moves Justin Holbrook's men to within three points of fourth-placed Wakefield.
Catalans, who had already been consigned to the Qualifiers, have now lost five of their last six games.
St Helens coach Justin Holbrook:
"It was a danger game for us especially after our win (against Hull FC) last week. We had to earn the two points and they pushed us.
"They started well and we were not up to our high standards in the first half.
"We have to give them credit. I'm really happy with the way we finished the game off.
"We swapped the lead a few times and the boys did well to close it out.
"We have got some wins together and it's a big game next Sunday (against Wakefield) and if we can continue to improve then I would certainly back ourselves (in the play-offs)."
Catalans Dragons coach Steve McNamara:
"It was disappointing to lose the way we did. With eight-and-a-half minutes left we were two points down on the scoreboard.
"It was a real tit-for-tat game. We found ourselves in a position to win it and we have conceded a lot of points at the end which makes the scoreline look ridiculous.
"We are going in the right direction and we know where we are for the rest of the season."
St Helens: Lomax, Makinson, Morgan, Percival, Grace, Fages, Richardson, Douglas, Roby, Thompson, Taia, Peyroux, Wilkin.
Replacements: Walmsley, McCarthy-Scarsbrook, Lee, Knowles.
Catalans Dragons: Gigot, Tierney, Inu, Duport, Yaha, Walsh, Myler, Moa, Aiton, Bousquet, Anderson, Horo, Baitieri.
Replacements: Casty, Bird, Da Costa, Navarrete.
Referee: Ben Thaler (RFL)

So what happens next?
If the politicians are not in a position to produce a budget, the civil service could take over.
Legislation allows the permanent secretary in the Department of Finance to take control if a budget act has not been passed at least three working days before the end of the financial year.
The official can access a sum of money not exceeding 75% of this year's budget and direct how it is spent.
If we get to the end of July and there is still no deal that official then has the right to spend an amount equivalent to 95% of this year's budget.
So that implies significant in-year cuts at Stormont unless a budget is produced by either an Executive or a direct rule minister.
This contingency plan was mooted during a previous budget crisis but was described by the then Finance Minister Arlene Foster as "dramatic and draconian."
The current finance minister, MÃ¡irtÃ­n Ã“ Muilleoir, has emphasised that the legislation means "monies will continue to flow".
However the TUV leader Jim Allister, who is also a senior barrister, said his reading of the law suggests it will not be so straightforward.
He believes that the Executive must at least issue a draft budget before the permanent secretary is empowered to take control.
Short of that, he thinks it could require emergency amending legislation at Westminster.
Whoever is correct, the budget uncertainty will have an impact.
Voluntary and community groups which rely on Executive will shortly have to put staff on notice of redundancy as they cannot be sure what funding they will get in the coming financial year.
A man and a woman have been critically injured in a three-car crash in County Down.
[NEXT_CONCEPT]
St Helens boosted their hopes of a top-four finish with a battling Super League win over Catalans.
[NEXT_CONCEPT]
The political crisis means that the Northern Ireland Executive still has not agreed a budget for the 2017/18 financial year.