"It's just tragic, based on the guns that are on the streets, that three young men have lost their lives today," Weis said.
"We're bringing out every resource we have, and we'll work this case as hard as we can," he said.
Officials hope at least two surveillance video cameras near the crime scene will lead to clues, said police Cmdr. Eddie Welch, who added that investigators are looking at "multiple scenes."
"They're in great positions," Welch said of the cameras.
No one was in custody as of Friday night, Officer JoAnn Taylor said.
The shooting occurred Friday afternoon about a block from Immaculate Conception School. It is not clear if any of the victims were students there.
One of the teenagers died at the scene, officials said. The others died after being transported to area hospitals.
University of Chicago Medical Center spokesman John Easton identified the 13-year-old victim as Johnny Edwards. The identities of the other two, ages 15 and 17, have not been released.
(This version corrects victim's age to 15, sted 14, per police.)
