Simplicity!! Pushing either of the A/P engage switches will (usually) engage all three AFDCs, left right and centre. The AFDC selection changes each time the autopilot engages - left or right the centre AFDC is used for monitoring and comparison, also the centre is not connected to any of the backdrive actuators.
Pushing the A/P disconnect switch on either column will of course disconnect all 3 AFDCs as will the disconnect bar on the MCP.
The MCP disconnect bar has three toggle switches underneath one for each AFDC.