The first alleged attack took place against a 13-year-old girl in a stairwell at Lord Nelson School in Vancouver in April 1995, according to police. The second involved two 14-year-old girls in Delta in August 2007.
The third attack was an alleged sexual assault on a six-year-old girl in Surrey in July 2009, while Hexaner allegedly ordered the girl's 12-year-old-brother and 15-year-old friend to lie on the ground.
In all three attacks, Hexamer is alleged to have threatened the victims with a knife.
Police said they used DNA evidence found at the three crime scenes to link Hexamer to the attacks.
Hexamer is facing a total of 23 offences, including several counts of sexual assault, sexual touching, invitation to sexual touching involving a person under 14, unlawful confinement, robbery and uttering threats.
Hexamer, who remains in police custody, has no previous criminal record, said police.