The trio will support Gabriel Byrne, the star of the series that centers on his character of a professional therapist whose demons only come out when he is on the couch himself.
Wiest, who won supporting actress Academy Awards for her work in the Woody Allen films "Bullets over Broadway" and "Hannah and her Sisters," plays the therapist's therapist and mentor who sees Byrne's doubts and anger bubble to the surface, The Hollywood Reporter said.
Underwood and George take on the role of two of Byrne's patients. Underwood plays a fighter pilot who killed three child hostages in a raid some years back, and George portrays a medical resident who becomes infatuated with Byrne's married character.
"In Treatment" is expected to air sometime next year.