I am assuming you have a pressurised system fed directly from mains water if your seeing 3 bar in the tank.
The most likely cause for a pressure drop is insufficient flow rate, into the tank, so you are pulling more water out than you can feed in thus pressure is reducing to compensate.
It may be worth checking all the inlet valves are open \ unrestricted, or put a service valve on taps to reduce the flow rate.
Please let me know if you need anything clarified.
The psychics behind water pressure is a little more complicated, opening 3 more taps wouldn't reduce the pressure by a 3rd, as you will probably find there is sufficient volume of water to maintain 2 bar for example with 3 taps open just not 3 bar with one tap open, you also have the benefit of gravity water on the ground floor (assuming the hot water tank is upstairs) will also be playing its part.
What you will find (as is more common on toilets) is the water is switched off over the next second or two the pressure builds ups behind the valve, so when you then turn on the valve its under pressure and you often get pipes which shudder due to sudden pressure differential.
A non return shouldnt cause significant flow rate issues but it has been known, my suggestion would be leave a tap on and measure the mains water pressure and see how many bar its at.
You can pick up a pressure gauge like the one here from somewhere like screw fix, and it just screws on the outside tap.
In terms of measuring without a gauge, you could measure flow rate but not pressure, but they are related. I would use a 2L coke bottle, that way your not taking 5 minutes to wait for a bucket to fill, and its a simple measurement unit (2L) if it takes 30 second to fill then the flow rate is 4L per minute.
The simplest way to stabilise pressure is place a pressure reducer on the mains inlet into the tank to drop the pressure to 2.5 bar which would make it less noticeable. Another options is to use smaller pipes \ install restrictive valves on the pipes feeding the taps, this will reduce the flow rate which in turn will maintain the pressure. In addition you could increase the diameter of the inlet \ feeder pipe into the tank, a 22mm pipe is the equivalent to two 15mm pipes, due to surface friction.
If this is causing significant issues, and your seeing the water flow rate drop to next to nothing this would suggest a blockage of some description, as this wouldnt impact pressure just flow rate.
Could there be a water pump which has tripped?
If the pump is to increase water pressure across the entire house it would be where the hot and cold water feed are, so probably directly after the hot water tank, probably spurred off the immersion circuit.
Photos probably wont help, but you can upload them here and share the link, https://postimage.org/
I am conscious this has turned into multiple questions, and I am limited to how much time I can spend on going back and forth with one question.
Clearly the sink is coming from the rising main and hence has a higher flow rate, the regulator you have is designed to reduce the pressure down, and the writing just says it needs to be calibrated before its used, the purpose of the valve is to stop the pressure building up in the cylinder.
The Bathroom basins are probably running off 15mm pipes so this may be one reason for the restricted flow, but to address the issue you need to get a plumber in who can intercept the pipes and different locates to measure flow rate and water pressure, and then see if there is a valve causing the problem.
My money is still on a valve which isnt fully open.
If your going to the effort of removing the valves I would replace them, at least you can then work back towards the taps once you have checked\replaced those valves.
My colleague also advised the model of pressure regulator you have (with the french writing) should have a strainer inside it, and this often becomes blocked, so would be worth shutting off the water at the stop cock and then removing that and giving it a good clean.
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