An N3 escort would make a convoy untouchable to German raiders

Would Bismarck or Scharnhorst ever choose to attack a convoy escorted by an N3 battleship?

No; an N3's nine 18-inch guns would overmatch either raider so completely that their rational choice would be to stay away.

For German commerce raiders, the matchup is a straightforward mission calculation. Ships armed with eleven- or fifteen-inch guns and ordered to avoid superior opposition would not willingly close an N3 carrying nine eighteen-inch guns. Deterrence is one of the convoy escort's effects: the battle may never occur because the attacker cannot accept it.