Officer Dora Fontaine was granted immunity for her testimony on Day 2 of the trial.
Ex-Officer Joseph Walsh, former Det. David March and suspended Officer Thomas Gaffney are accused of falsifying their reports to cover-up for officer Jason Van Dyke who was seen shooting McDonald on dashcam video.
Fontaine said she witnessed the shooting, and said that details in reports that were attributed to her not accurate, including the detail that Van Dyke was the victim of assault.
She also testified that officers in the department have called her a "traitor" and are refusing to back her up on the street.
The trial comes after Van Dyke, a now-former Chicago police officer, was found guilty of second degree murder for shooting McDonald 16 times in 2014.
The three officers are accused of coordinating their stories and making false police reports.
This is a bench trial which means a judge, not a jury, will decide the defendants' guilt or innocence.
The trial is expected to wrap up next week.