Police say the crash happened on Regional Road 20, near Pelham, Ont., at about 1 a.m.
"The accident happened on a bit of a bend in the road," said Const. Rich Gadreau of Niagara Region police. "It's a country road, it's one of the main arteries through this area."
Police believe the westbound vehicle drove into the eastbound lane and the cars collided head-on.
"One of the vehicles ended up in the southside ditch, the other one on the other side of the road," Gadreau said.
Three people died. Five others were airlifted to hospital.
Gadreau said there were four people in each of the cars.
All three who died were in the same car.
Police have not released any information on the identities of those involved in the crash.