Three students at a Brooklyn middle school were arrested for allegedly sexually assaulting a 12-year-old girl in a basement stairwell last week, police sources said.

The students at Mott Hall No. 4 Middle School in Brownsville allegedly pulled down the victim’s pants and forcibly touched her at 4:30 p.m. Thursday, sources said.

The principal reported the attack to police Friday night.

Two of the boys, ages 14 and 12, were arrested later Friday. A third, age 13, was arrested Monday.