Michael J. Grennan, 33, of Loveland was arrested after the Saturday accident and charged with careless driving resulting in death, a misdemeanor, the Thornton Police Department said in a release.
The Denver Post reported police said the victims -- two men who died and a teenager who was critically injured -- were hit Grennan's pickup drifted onto the dirt shoulder of a road.
Police said Grennan pulled over and called for help.
The newspaper said other drivers told police they saw the truck drifting before hitting the pedestrians.
Notable deaths of 2014 [PHOTOS]