I'm not against any particular formation, I've seen them all since the 1950s and if you have better players than the opposition most can work. However, I feel at the moment we don't have the 3 quick and quality centre backs for KCs system. That can change of course and one answer may be for Hart to play there similar to how Kyle Walker plays for England. You are then losing a good left full back so it's a case of what fits best.
McGahey would be better in the centre of a 3 IMO with Pidge and Hart wide of him.