The incident happened on February 13 at 7 p.m.
Surveillance video shows teen walking on the 1800 block of South 2nd Street and being followed by the group of men.
Police say one of the suspects then stepped in front of her, pointed a handgun and demanded her cell phone.
The trio then fled north on 2nd Street before crossing to Moore Street.
The first suspect is described as a black male, 17 years-old, 5'11"-6'0" tall with a dark complexion, dark colored eyes and wide nose. He was wearing a blue ski cap and blue jean jacket.
A description of the other two male suspects is unknown.
Anyone with information should call police at 215.686.TIPS (8477) or text a tip to PPD TIP or (773847).