I did it, it was easy enough. With the Focus, to go from 2 to 3, you need the screen, APIM (controller module that attaches to the screen), a newer USB socket in the armrest for Carplay, and a replacement GPS antenna (Sync 2 uses a Canbus-interfacing antenna, 3 uses one that directly connects to the APIM via a Fakra connection).
Then it's just a case of disassembling the centre console, swapping the bits over, finding a home for the GPS antenna (I just swapped out the Sync2 one in the box behind the rear view mirror) and using Forscan to match the config of the APIM to what your car has and hasn't got fitted (reversing camera/parking sensors/climate control and whatnot).
Just found this:
https://www.youtube.com/watch?v=M_AuGTOeO1A
Yep, same procedure.