Medical officials said the girls, all 15, received glancing blows by the bus, the Australian Broadcasting Corp. reported.
Soon after classes were dismissed, an unattended parked bus rolled down a hill outside Pacific Hills Christian School at Dural, The Sydney Morning Herald reported. The bus ran into a group of students before crashing into a tree.
A police investigation has begun.
Two girls were taken to hospitals, officials said. One girl suffered injuries to her stomach, leg and neck, while the second was treated for leg and shoulder injuries.
The third teen was treated at the scene.
"The initial fear was for the worst," Ian Badham, director of CareFlight, which sent paramedics to the scene, told the Morning Herald. "But it knocked them down with relatively minor injuries considering the potential for what could have happened."