ackie Healey, 23, and her colleague at the Behavioral Health Foundation were severely injured after the attack on May 29.
The latest suspect has been charged with aggravated assault, robbery, unlawful confinement and theft of a motor vehicle.
All three suspects remain in custody.
The other 16 and 17 year-old have been charged with aggravated assault and forcible confinement, as well as possession of a prohibited weapon, possession of a weapon dangerous, and theft of a motor vehicle. These two teens were clients at the centre when the two women were attacked.