In the 757:
There is a pressure port on the back of that panel with the 3 indicators (it's officially called the press and ind warning module). The pressure port provides the cab alt and cab alt rate info. The diff pressure comes from a differential pressure sensor below the cockpit floor (the elec equipment bay). That sensor gets static pressure from the alternate static ports (the ones that also provide your standby instruments) and also senses cabin pressure. A diaphragm in the sensor then differentiates the two inputs to indicate your differential pressure.
The two ADC systems come into play for the auto operation of the pressurization, not for the 3 indicators you mention. (1 ADC inputs into each control computer, and the computer itself has a pressure port on it's face for cab pressure).
Then there is also a cab alt warning switch below the cockpit floor, that provides your EICAS warnings+ aural+ the dedicated lights next to the 3 indicators + master warning lights.