The accident occurred as the young boy was being pushed in a stroller by his older sister crossing the street at North 20th Street and East Camelback Road in Phoenix.
"They were crossing the crosswalk with other family members. There was the mother, the children's grandmother and another sibling," Phoenix Police Sgt. Steve Martos said.
As the family was crossing the street, Martos said they believe a pickup truck travelling northbound on 20th Street made a left turn onto Camelback Road and struck the boy.
"The child became lodged underneath this pickup truck," Martos said. "When our officers arrived on scene they immediately began CPR on this child."
Martos said a bystander, who is a physician, also tried to assist officers as they gave medical attention to the boy.
The Phoenix Fire Department arrived and transported the child to a nearby hospital where he was later pronounced dead.
Martos said the investigation will continue for some time and police have detained the driver of the truck who stayed at the scene after the collision.
"We have currently a lot of restrictions in that particular area until we can conclude our scene investigation," he said. "We have detained the driver at this point and are currently going through the process of determining if in fact the male driver was impaired during this collision."
Martos said no one else was injured and the family was crossing the street inside the crosswalk. He said they also believe the family had the right of way and were not crossing against the light.
"It's an unfortunate situation," Martos said. "We're talking about a family … likely shopping for the Christmas holiday and unfortunately now they have a 3-year-old boy who has lost his life."