My water gets very hot no matter what I switch on the thermostat setting, I've changed the cylinder thermostat, that didn't work, now I need to know how to check the three port valve actuator or synchron motor to see if it is working correctly , has anyone got any comments please

Would this be a gas or electric water heater? Back to you, Tom

If your three port valve actuator is a MIXING or TEMPERING valve then that could be an issue, for sure. Otherwise, I don't see how a three port valve actuator or a synchron. motor could have much to do with temperature setting of the heating cylinder?? With that being said, I'm from the USA and may not understand your system well enough to comment.

It seems to me that the issue is with the thermostat...could have purchased a defective one, I guess, or that three port valve actuator is a mixing valve and, if you are lucky, you can replace the thermal element inside the unit.

Mark

Not enough information is given to give a proper review and offer a solution. It appears from the terminology you are using you may be in another country. (i'm in the usa). My comments are UNDERLINED to differentiate from your comments: you wrote:
My water gets very hot no matter what i switch on the thermostat setting, (what thermostat setting are you referring to? Is this a thermostat setting on a water heater or boiler or is it a thermostat setting on a thermostatic mixing valve or other type of temperature controller? Need more information on make and model and description of the device or product)
i've changed the cylinder thermostat, (if by cylinder you are referring to a water heater tank then the question is: is the water heater gas or electric? If it is electric, the electric water heater thermostat is typically a thermostat that is mounted to the exterior of the water heater tank. If it is not the right size or if it is not clipped back into the thermostat mounting bracket properly, a space can be left between the thermostat and the tank wall. That space can result in a heat transfer problem from the tank wall to the thermostat and the result is an overheating tank.)
That didn't work, now i need to know how to check the three port valve actuator (I assume by three port valve you are referring to a thermostatic or temperature actuated mixing valve)
Or synchron motor
("SYNCRON MOTOR" is not a term used in the united states related to plumbing. Not sure what you mean) (in the usa we refer to the temperature control in a three port valve as the thermostatic element or thermal motor. We also have several temperature control valve standards with ASSE and CSA that the valves must be tested to) to see if it is working correctly , has anyone got any comments please