Grand jurors charged Jamelle Andre James, 19, Robert Lee Morris III, 19, and Sir Marcus Winfrey, 22, all of Mount Morris Township, with possessing stolen firearms and being unlawful users of marijuana in possession of firearms, U.S. Attorney Saul Green said.
Winfrey is the uncle of the 6-year-old boy police say fatally shot fellow first-grader Kayla Rolland on Feb. 29 in a classroom in Buell Elementary School in Mount Morris Township, near Flint.
James, a friend of the boy's family, had earlier been charged with involuntary manslaughter in Kayla's death.
Authorities allege he gave the boy access to the handgun used in the shooting, a stolen .32-caliber semiautomatic.
Winfrey and Morris were not charged previously, but Winfrey has been in custody since the shooting on unrelated charges. Morris was arrested Thursday, authorities said.
Genesee County prosecutors say investigators believe the gun had been left under James' blankets at the house where the boy was staying with his uncle.
Authorities say the boy told them he took the gun to school to scare Kayla, with whom he had apparently quarreled, but ended up killing her with one shot.
Prosecutors have said they will not charge the boy because he is too young to understand his actions.
In a statement Thursday, prosecutors alleged that Morris sold the stolen pistol in December that eventually wound up in James' and Winfrey's possession. Morris also was accused of selling a stolen 12-gauge shotgun in January.
Prosecutors allege that James and Winfrey knew the weapons were stolen.
Authorities say they found the same stolen handgun in Buell Elementary shortly after the shooting, and searchers found the shotgun later that day in the flophouse'' where the boy had been staying.
Each charge carries a sentence of up to 10 years in prison and a $250,000 fine.
Rolland was killed by a single gunshot.
The boy accused of shooting her, his brother and their 5-year-old sister were placed in the custody of an aunt.