It happened just before noon Tuesday on South Street in Danbury.
Police say the driver, 45-year-old Edward Kish, lost control of his car, crossed the median, and struck another car.
The impact forced the vehicles into a box truck.
Both drivers, Kish and 78-year-old Jane Shannon, were killed in the initial crash along with her passenger, 77-year-old William Shannon.
The truck's driver, 59-year-old Roy Bass, was not hurt.
