It's interesting seeing so many believe that van Gaal will actually go three at the back even after what van Gaal said and the fact that we only have 4 proper central defenders in our team (I'm counting M. Keane as a first-team defender at this point). Also, the midfield plays a very important role in a 3-man-defence system. The midfield determines whether the team sets up for counterattacks, seeks to maintain possession, or be all-rounded and dynamic. However, the 2 deep midfielders must be solid defensively; otherwise, we'll be easily opened up through the middle. Plus, the fact that this setup requires wing backs to push up to provide the width means that the opposition can easily stretch the 3-man-defence setup and create gaps in the middle (as Australia demonstrated against the Dutch).
For us to implement a 3-man defence setup, we'll need more central defenders and more central midfielders who are defensively solid. The midfield needs to be able to cover up any of the defensive gaps that form whether out wide or in the middle. Also, we'll need one of our front 3 to drift out wide in order to prevent the central areas to be heavily congested (van Persie can do this excellently as well as Mata or Kagawa).
Ultimately, though, I doubt we'd go for a 3-man-defence setup given what we have right now. Unless we get 2 central defenders, get 2 defensively-capable central midfielders, and sell all of our wingers bar Januzaj, we won't play a 3-5-2 or a 3-4-3. Besides, van Gaal has the freedom to buy any player(s) for his favoured setup rather than not as is the case with international football.