The incident occurred around 7:15 p.m., where the Fairfield-Suisun Unified School District was hosting a graduation ceremony for Sem Yeto High School.
Police say the incident remains an active scene and no suspect is in custody.
There is no information about the victims and whether they are students.
The three injured are being treated at a hospital and their current condition is not known at this time.
Stay with ABC7 News for the latest details on this developing story.