What temperature is the boiler output set for? Some boilers have a single boiler temperature setting for both HW and CH, others have separate settings for each mode.
To test the 3-port valve operation, the OP simply needs to turn on and off, in turn, the HW, then CH. He should then hear the actuator react, to the changes in demand. If the suspicion is that the actual valve itself might be stuck, and the actuator head is removable - he could remove the actuator and try turning the valve spindle back and forth. It should almost turn, with finger pressure, but a pair of pliers might be needed.
If the actuator head, is not responding by moving, a common fault, is the motor failing. They can be bought quite cheaply, but not easy to fit, simpler to just buy a replacement actuator head.