The OKW StuG III voice is actually used when they call in a Sturmtiger, which implies the voice acting for planned units was done before they scrapped the StuG for the Sturmtiger. Really odd, they could have used the "Tiger" voice for the Sturmtiger instead, or a combination of the two. I like OKW's original design, the inclusion of the StuG seems to imply it was more generalist and cheap than the strange combination of starved and specialist.
Pz III. 
There are actually files for a Panzer III, exact model isn't clear but it was at least a Model L as the gun is named panzer_iii_50mm_mp.xml. Stat-wise, it seems it was intended to be a counterpart to the T-70. Possibly dropped because Relic couldn't spare the resources to make it, it uses the Panzer IV Ausf. F1 model and is not functional as a unit, although if spawned it will shoot at enemies.
why the fuck werent these released
Because design changes or balance problems.