I agree with Nyama. The ARMA III BTR-K Kamysh is obviously (very) loosely based on the Ukrainian BMPT-64. Just for the reference of others I've included two images, one of the actual vehicle and another of the optional modular turret seen in ARMA III.

Additionaly, the Russian phrase for BMP, or Боевая Машина Пехоты readily translates as Infantry Fighting Vehicle.

This article from Dyslexi covering basic controls in ARMA III should clarify the issue somewhat. I draw your attention to the section titled "Default Stances" and how incremental postures go beyond just standing, kneeling, and prone.

http://dslyecxi.com/arma-3/arma3-alpha-controls/

Personally I found a keypad thumb-stick like that found on the Razor Nostromo to be invaluable to being able to adjust stance in small stages. OH and having a TrackIR for head movement goes a fair way as well ;)

Cheers