Police originally reported those fatalities had occurred on Dances Road at Caboolture, but later amended information to say they'd been swept off Beerburrum Road.
A man in his 70s, a woman in her 30s and an eight-year-old boy perished.
Police are investigating whether the vehicle drove around a road closed sign.
Moreton Bay mayor Allan Sutherland told ABC radio indications were that that was the case.
"If there's a sign up and it says the road is closed, it means the road is closed," he said.
"We don't close them for the fun of it."
In a separate incident, 75-year-old man was swept to his death after a car was inundated at nearby Burpengary around 8.30pm on Friday night.
Police and a swift water rescue team were called to Morayfield Road, near Burpengary Creek, where they rescued a 68-year-old woman clinging to a tree.
The man's body was found at 3am.
Most areas in the southeast received over 100mm of rain across Friday into Saturday.
But 277mm of rain was dumped on Caboolture in just three hours.
Southeast Queensland's beaches were pounded by monster surf, with a 13 metre swell recorded at Tweed Heads at one point.
A severe weather warning remained current for dangerous surf on the Gold and Sunshine Coasts on Saturday.
- AAP
