Pzkpfw III Ausf G-L is Germany's standard "light" tank. Its full name is the Panzerkampfwagen (Armoured Fighting Vehicle) III, with Ausf (for Ausführung) denoting its variant, which in Spring: 1944 is represented by versions G, H, I, J, K, and L, all of which are similar enough to be grouped into the same in-game unit.
The Panzer III was designed starting in 1934 as a medium combat tank, armed with a 37mm high-velocity anti-tank gun and used primarily as a combat tank. During the invasion of the Soviet Union and fighting in North Africa the Panzer III was found to be insufficient for this role and was replaced by the long-barreled Pzkpfw IV and reclassified as a light tank. Re-armed with a 50mm high-velocity gun, the Panzer III served the rest of the war as a hybrid light-medium combat and reconnaisance tank.
Compared to its closest contemporary, the Soviet T-70, the Panzer III is better armed and armoured; indeed its actually better armoured than the Panzer IV (at least on its vulnerable side). However it is still vastly underprotected and underpowered against medium tanks of other nations; its best use is as a light support tank, engaging enemy light tanks, armoured cars, vehicles and self-propelled guns.
Keep in mind that the Panzer IV is only slightly more expencive, while actually being a medium tank. Panzer III's are made only in two situations: the weapons of the Panzer IV does not matter, as the tank is build just for the purpose of having an armored unit on the field, or the gun does not matter as anything the enemy has is massivly outclassed by the presence of a tank factory in general (like trucks). In all other cases, making a Panzer IV is a better choice, as you get a general-purpose unit instead of relativly expencive anti-infantry support.