Need your inputs regarding Pressure measurement

Hi there!

I have a Hydraulic circuit that is meant for pumping water to a certain height. There are actually three pumps whose outputs are coupled together as one and sent to a vertical tube.

The problems is with the pressure sensor output. It is well known that the Pressure is an INTENSIVE Property. But the output curve shows me that pressure is added together one over another upon adding three pump outputs together. Being an intensive property it can not be so, in my opinion. It should not get added up. How can that be possible? Where have I gone wrong? Please help me out!!

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Circuit

Input signal to valves as steps. Each curve is fed into one of the valves as can be seen fro the figure

Pressure sensor output