Triple redundancy is not really the standard, considering that the most common types only use two (737, A320, both CAT IIIb approved).
On somewhat older 737s you could feel in the controls the second autopilot engaging after the self test around 1500ft AGL was passed. I don't think it is really passive. Even more interesting is the single channel autopilot hardover in the simulator compared between one autopilot engaged and two autopilot engaged.