It is not something that can easily be described over the internet.
however.
if you have a permanent live and a neutral then you are able to power your new thermostat.
that would use 2 of the 3 cores you have available.
If the boiler uses a 230v signal to provide heat then you can use a link from perm live to the relay and the 3rd core to return the signal to the boiler.
however, if the boiler is not using a 230v return from the thermostat, you are likely to destroy the boiler control board.