Tonic
Some earlier Classic B747 did, indeed, only have two autopilots.
Except for a coupled approach, consider each A/P as a separate and independent system, all have the same functionality.
When coupled and all engaged ( I forget the height this happens) the system logic has the three systems cross voting, if one is out of whack, it will disengage.
The memory aid: Three up, Cat 111, two up, Cat 11.