Police said an officer shot a 13-year-old after the boy turned toward officers with what looked like a Beretta handgun. It was a pellet gun made to look like a Beretta.
Because of the teen's size, 5 feet 7 inches tall and weighing 200 pounds, the officer believed he was dealing with a young adult male.
The boy was transported to an area hospital where he underwent surgery and was listed in critical condition.
The two other boys were detained without incident after dropping their replica guns. In total, three replica guns were recovered.
The shooting remains under investigation.
The officer involved was identified as Victor Abarca, who has been with the Los Angeles Police Department for two years and 11 months.
City News Service contributed to this report.