The Norway IC supplies its own troops to fight off nasty incursions from east and west and
bringing pressure on northern USSR while the airbase brings extra scramble ability to SZ112 fleet and the ever hopeful Sealion - admittedly not turn 3-4 but maybe later. Still thinking it through.
You will have no choice but to reinforce it sometime by sea or air because once all three allies come down on it and they will, I promise you ten infantry per turn is nothing.
If all 3 allies are attacking norway you have other problems with germany then keeping norway.
Lets just asume that france and anzac are not part of those 3 so that means USA/UK and Russia, if russia is having fun in the baltic your in deep shit