In general, the core structure for an infantry battalion TOE (Table of Organisation and Equipment) from WWII onwards looks something like the following:
1 Headquarter company
0-1 Combat support company (AKA heavy weapons company)
3-4 Infantry companies (AKA rifle company)
Headquarter company contains administrative, signal, logistical and maintenance elements. While most of these are not displayed in game, ammo carrier section/platoon might present part of the HQ company ammunition platoon. Still, the men of the HQ company do have small arms for defense should the situation get desperate (and thus could be displayed eg. as reserve platoons in scenarios).
Combat support company contains platoons of various heavy weapons, such as mortars, machine-guns and various anti-tank weapons. It might also contain an engineer platoon and/or reconnaissance platoon (though the later might also be found in a HQ company). In battle, these could be held in reserve or deployed to the for to support individual rifle companies.
Infantry companies are your usual line companies, typically containing 3 infantry platoons and 0-1 heavy weapon platoon. Typical battle deployment would involve one of the companies being held in reserve.
Reconnaissance companies are something that you would not normally find in an infantry battalion - they are likely to be higher level elements (eg. part of divisional reconnaissance battalion).
I am afraid I cannot point any specific websites for modern TOEs, but the following website is excellent source for WWII battalion organisation, and would probably form a good guideline for immediate post-WWII period too:
http://www.bayonetstrength.150m.com/
Griefbringer
Re: What 3 companies should I buy If I want to be as accurate as possible?
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