The stabbing happened in midtown Manhattan Wednesday, just around the corner from the new Broadway production of "Groundhog Day."
Police say the suspect is a 16-year-old boy in a red hat, and he fled.
Authorities say the victims are a 16-year-old boy with a stab wound to the stomach, a 17-year-old boy with a slash on his ear and an 18-year-old with a wound on his arm. Authorities say the victims suffered non-life-threatening injuries.
A call to the city's Department of Education wasn't immediately returned.