Prosecutors allege the men sexually assaulted the girl 135 times while she was aged between six and eight years old, broadcaster WDR reports.
One of the defendants is the girl's uncle, another is her older brother, while the third man is her mother's partner.
The girl's mother is also on trial for assisting in the crime.
The girl, now 17, has been in psychological care for years. She is set to give a statement during the trial.