Police Lt. Ozzie Valdez said the girls — ages 13, 14 and 15 — were charged in Sunday's attack. Two of the girls have been charged with aggravated battery and the other charged with mob action.
Police said the teens who attacked the girl knew the victim before the attack, and she believed they were her friends.
"It broke my heart to see this group of young people turn on her and escalate it into what we saw, a physical assault," Police Supt. Eddie Johnson said in a press conference Thursday at Chicago police headquarters. "There simply cannot be room for this type of divisive and hateful behavior."
Video of the beating was posted on social media, and Chicago police began investigating after CBS Chicago sent them a link to the video.
Valdez has said the girl met with several teens who "she thought were her friends" on the day of the attack. Those friends taught her how to flash gang signs before the beating.
A family friend has said the girls who attacked the victim were upset that she did not want to participate in a sex act they were planning and that she planned to report it to police, according to CBS Chicago.
Johnson did not specifically confirm that claim.
"There was some things going on that these young ladies wanted her to do that she didn't do, but we'll leave it at that for now," Johnson said Thursday.
Police have said they also are investigating whether the girl was sexually assaulted, based on her statements to detectives, but Johnson would not discuss that investigation.
"We're not prepared to talk about that at this time," he said.
The girl's father reported her missing the day after the attack. Hours later police learned about the video and launched a criminal investigation.
"If you're dumb enough to post a crime on social media, that just shows you where your head is," Johnson said.
In the video, the victim is seen trying to defend herself as another girl continues to corner her and hit her — with the help of others.
"One of her friends hit her in the face, striking her in the head, that left lacerations on her forehead," community activist and family friend Stringer Harris said Wednesday. "Another friend attacked her brutally and beat her and another gentleman was there as well."
The victim's grandmother said the girl's attackers must face consequences.
"I'm mad that they took advantage of my baby like that," said Charlie Mae Harris. "They need to get off the street. If they did it to her, they'll do it to someone else."
The girls who were charged in the case turned themselves in to police with their parents Wednesday night. Johnson suggested they also provided police with information about others seen in the video.
"Some of the parents of the offenders and the offenders themselves also did the right thing and brought information and individuals forward to police," Johnson said.
Valdez confirmed detectives are investigating other people involved in the attack, but would not say how many.
On Wednesday, police said the girl had been missing for five days before her family filed a report, but police have not explained why the girl's family waited so long to report her missing.
The girl was located Tuesday night, when a friend found her on a bus. The friend contacted the teen's family, who took the 15-year-old to the police station. She later was treated and released at a hospital.