Desirée Mariottini’s body was discovered in a building used as a squat in the Roman district of San Lorenzo on October 19, two days after she called her grandmother to say she’d missed the last bus home and would be saying with a friend overnight.
Investigators believe she fell unconscious for several hours after being given drugs by several people, and died of an overdose.
She was sexually abused more than once before she died.
Two Senegalese men aged 26 and 43 were held overnight on Wednesday, and a 40-year-old Nigerian was detained on Thursday.
It is expected they will be charged with group sexual violence, voluntary manslaughter and drug dealing.
All three suspects reportedly have previous convictions for drug dealing.
Police are still searching for a fourth suspect, according to Italian news agency ANSA.
Mariottini lived with her mother, grandmother and a sibling in Cisterna di Latina, south of Rome.
“I want justice for Desirée. I don’t want this tragedy to happen to other girls,” her mother told reporters outside the family home.
Deputy Prime Minister Matteo Salvini declared that the girl’s murder would not go unpunished.
“I will do everything so that the worms guilty of this horror pay in full for their vileness, with no discounts,” he said.
Meanwhile, Mayor of Rome Virginia Raggi, announced a day of mourning for Rome to coincide with Mariottini’s funeral, the date of which has not yet been announced.
Raggi took to Twitter to promise bolstered controls in San Lorenzo, an area known for drug dealing.
She also announced plans to demolish other squats.
