The shooting was not intentional, Sgt. Corbett Ford said Thursday.
After the shooting, the surviving teens were taken to the police station for interviews. The two girls were later released.
The two boys were arrested on suspicion of possession of a stolen firearm; one boy also was arrested on suspicion of negligent homicide.
The parents of one of the boys, who lived at the home, reportedly were upstairs when the shooting took place.
The gun was believed to have been taken from a neighbor in the past few weeks, during a rash of residential burglaries in the Cottonwood Heights area, Ford said.
The Tribune is not releasing the victim's name pending police confirmation.
A friend of the victim said the boy was "always there for everyone."
"He was one of the coolest kids I knew," said Chey Woodside, who will be a freshman at Brighton next year. "He was ... just a total sweetheart to everyone. He always set his problems off to the side to help others. He was a great kid and he'll be dearly missed by everyone."
Ricardo Logan had been friends with the victim since seventh grade.
"He was a pretty mellow kid," said Logan, who, along with the victim, had just finished his junior year at Brighton. "He didn't hold grudges against people. He was down for whatever, he'd want to go for a walk, want to go for a hike.
"He was an overall great kid."
Logan said he was stunned Thursday morning to learn of his longtime friend's death.
"You can never expect a kid or anyone passing that way," he said.
Brighton High School released students yesterday for summer break, but counselors and a school psychologist are available at the school to any students who are struggling with their classmate's death, said principal Charisse Hilton on Thursday.
"We're just really concerned about the kids in our community and how this will impact them," Hilton said. "It's always hard when we lose a student, regardless of the circumstances. And it has a big impact on our teachers. They really do care about these kids."
Police said they were withholding the name of the victim pending notification of family.