What 3 companies should I buy If I want to be as accurate as possible?

[Marcello]

Globalsecurity is a good source but there is a lot stuff if you do not know exactly where to look at.
Here is a list of current US Army field manuals.

http://www.globalsecurity.org/milita.../fm/index.html

Some of them deal with battalion/company operations and organization. You can find them using the seach function in your browser (try "company" or "battalion" in the query).
FM 3-21.11 THE SBCT INFANTRY RIFLE COMPANY offers for example a very detailed breakdown of a Stryker brigade from the brigade level down to the single infantryman

http://www.globalsecurity.org/milita...m#sectionii1_5

That being said paper organization is one thing and reality is an other. For a start the practice is to raise ad hoc task forces on the basis of what is needed or available for a specific task. Therefore to make a real world example from the Iraq war a tank company might get attached to a Stryker battalion to stiffen it up.
Then attrition takes its toll.Again in Iraq a tank or two in a platoon might break down and the crews get issued armored cars and are used as infantrymen.
These are only two examples I remember but the point is that the combination of the mix & match system and attrition would produce a wide range of non standard formations.
I would bet for instance that there is not a single pure paper strenght Abrams battalion in the entire iraqi theater.