I just dont see why Van Gaal would be committed to us playing with 3 at the back, it isnt an important component of his "philosophy", it isnt something he has played with often in his career and he has not been such a strong advocate of the system that he stands to lose face by abandoning it. He said our squad lacked balance and we had too many number 10s and not enough - or perhaps insufficient quality - defenders and defensive midfielders. He has now at least partially rectified that. With the new additions he has made he has various other options he didnt have before which he can now experiment with. If he can find a shape the players are more comfortable with, enabling them to focus on learning how he wants to play (again, distinguishing, as he does, between the formation and the philosophy, which are two entirely different things) it would make a lot of sense to do that.
Maybe he sees learning the 3 at the back system as a useful case study in tactical flexibility, maybe he thinks it will help the players develop, maybe he has even decided in the twilight of his career that this formation is particularly strong and is not just a way of masking the limitations of a weak squad, as it seemed to be with Holland and with us when he first joined. But I just cant see it.
I have no idea if Van Gaal is the kind of person who cares what other people think, whether he would keep doing something because he wouldnt want to be seen as backtracking. But whether he does it for his own reasons or for perception doesnt matter, he has an out either way. If its all about what he thinks, he now has more options. If its all about what others think, the arrival of the new players changes the facts sufficiently that he can deny he has done a U Turn. Whichever it is, he now has the options and the excuse to drop it and move to 433 or 4231.