The black wire is neutral, it is not needed, the thermostat will work without it, however with it connected it will reduce the hysteresis.
There is nothing to identify the two red wires, one is feed in, and one is feed out, if the wrong way around the hysteresis reduction will not work as it should, but thermostat will still work.
If the black neutral wire it put in wrong place it can cause damage. So if not sure, don't connect.
So old black should go to N and other two to L and 3, So 2 = N, 1 = L and 3 = 3.
Personally I would use Flomasta 22199SX Wired Digital Programmable Room Thermostat (6259G) screwfix number, it is two wire so black not used, just the two reds, and it does not matter which way around, and it also has a programmable function.
Even with nothing connected the new thermostat should click,
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