BMP-3 can't use in game 2 extra MG. All slots full
I think it's more a "balance" problem : There is no coaxial MG in the game, and sometimes independant MGs aren't here (for example the tank destroyers using optional AA MGs, or the first versions of the Marder IFV, even if there is enough slots).
There can be many reasons :
-the 5 points increment cost system, because a coaxial MMG is not very useful (can't be used against helos, have the same range as RPGs, can't be fired at a different target). That's the reason why the Skot 2A is the only vehicle with a KPVT and a coaxial PKT, even if the BRDM 2 and the BTR use the same turret.
-Locking weapons in a group composed of many units with 3 or more MGs each is... not relaxing
-It's outside the scope of the game (like firing ports, it would add much confusion to the game for little or nothing in term of effectiveness).
The lack of MGs makes AML 60 sad