There are two configurations for Nest Gen 3 with respect to setting hot water temperature. Both require the interface to be Open Therm and are dependent upon the boiler manufacturer and model having Open Therm that is fully compliant with Nest. Open Therm is not the universal standard it was intended to be!!
The two settings ( to control hot water temperature) are Combi and System ( hot water storage tank). Generally Nest will function with a conventional Combi boiler, allowing the hot water temperature to be set from around 40 degrees C upwards. For storage Combi, it is a bit hit and and miss. The temperature received by Nest is only when the boiler is delivering hot water in combi mode - it (Open Therm) does not communicate stored water temperature ( in the Combi store boiler).
For a system boiler to work with Nest Gen 3 to control hot water temperature, there needs to be a separate stored hot water thermostat input into Open Therm. Open Therm then transmits this data to Nest. Be aware that, in this mode, if you set a high hot water temperature that is above your boiler flow temperature Nest will not recognise this, so does not adjust the boiler flow temperature to suit. Nest operates purely on an on/off basis until the programmed temperature is reached.
Note in system mode, Nest will not let you set a hot water temperature of less than 55degrees C.
In simple terms, most people set a hot water temperature once and then leave it - they tend just to want to be able to control the timed functions. This is often more easily (and cheaply) achieved through direct controls at the boiler / tank thermostat. Nest (and Honeywell Evohome) make it overly complicated to display a hot water temperature
Nest has developed significantly since it’s introduction in the UK, consequently ( particularly with Open Therm control) it is not always a straightforward installation to achieve its full functionality.