I am over 45 years from my fluid mechanics classes so it take lot of nut oil to bring those memories to the top.
One more time from the beginning: When there is no more flow of (incompressible in theory) hydraulic fluid, brake pads are pushed against rotor, the hydraulic pressure depends from force used on pushrod and size of master cylinder piston. The pressure in line and caliper pressure chamber reaches 500 PSI. Then, what is a pressure per piston? Is it "divided" by number of pistons in chamber? 2 pistons = 250 PSI. 3 pistons = 167 PSI per piston?
What did I miss here?
BTW, how do you deal with increased movement of master cylinder piston when you changed to 3-piston caliper?