I have a Drayton mechanical room thermostat and I want to replace it
with a programmable digital thermostat.
The Drayton had three wires connected to its three terminals numbered
1,2,3.
My CH system has a boiler indirect I think a HW CH programmer and a
three port motorised valve that seems to be controlled by the room
thermostat. the problem is that the Drayton seems to have seized up
and will not cut off the heating . that's why i want to replace it.
The new one I've bought has also three holes named NO,COM,NC.
What wires do i connect to what terminal?
Please help

I do know that the voltage between 1 and 3 is 230 V and also between
2and 3 is also 230 V.