Trev,
Getting the MAC direct from the Raspberry Pi is not my area of expertise, but if it helps, what we need is the 12 character MAC Address which will start B827EB.....
For 'non-techies', the normal way to determine the MAC address is to put the microSD containing the PilotAware Software from your existing PAW into the 'new' Pi3, power it up and watch for the WiFi to appear on your phone or tablet WiFi list. You won't need a WiFi dongle with the Pi3 as this is contained on the board (or any of the other 'dongles' at this stage). Once the software has booted, the MAC address should appear automatically as part of the 'new' WiFi address.
Hope this helps.
Regards
Peter