When you get the snowflake symbol on the dashboard display indicating it is less than 3°C what functions are automatically started on the car?
Absolutely none. The snowflake is just a visual warning to indicate that there may be ice, and/or black ice on the roads - and that you "should adapt your driving style accordingly".

I'm specifically interested in whether wing mirror heating is turned on?
Nope, as others have said, door mirror heating is ONLY activated by rotating the knob on the drivers door to the correct position. However, be WARNED, the mirror heating is one of the first things to be over-rided and automatically turned off when the ECU senses a too high an electrical demand.