van Gaal wont play 3 in midfield all the time.
I expect he will switch formations and tactics to suit. That is why Rooney, Van Persie, Mata and other wide players being mentioned will have a role in his team.
Van Gaal nearly always plays with a number 10 in his side, be it in his traditional 4-3-3 formation or be it in his new experimental 3-5-2 formation. Mata is the logical choice to be that number 10, he is the one with the best passing skills in our team and atm he is our record transfer signing, you can't possibly be serious thinking LVG would leave him out in his XI.
----------------DDG---------------
Rafael-----X----------X--------Shaw
----------Herrera---Vidal-----------
---------------Mata----------------
Cuadrado---------RVP--------Januzaj
or something like
---------------DDG--------------
---------X------X-------X-------
Cuadrado---Herrera--Vidal----Shaw
----------------Mata-------------
------------RVP-----Rooney------
are the sort of systems I envisage with LVG (Cuadrado (Di Maria could be an alternative) ofcourse being an assumption, just like Vidal)
Point is the midfield always looks the same, as it is the hearth the of the team. He might play 4-3-3 against some lower teams where he wants to go all out attacking, he may play the 3-5-2 in games were defence is more important. Rooney is a golden sub in the first system here he can be an alternative for both Mata and RVP.
no need for somebody like Carvalho in these systems, not if you get Vidal. He could be an alternative to Vidal however, could play like this:
-------------DDG------------
Rafael----X-------X------Shaw
------------Carvalho----------
-----Herrera---------Mata----
Cuadrado-----RVP------Januzaj
But 3 new midfielders is just overkill and won't happen if you cosnider the fact how much money they would cost and the fact we also still need to aquire a new winger (Cuadrado, Di Maria or Sanchez) and a Centreback. We can do with one more, but two more is overkill and a waste of money.