“Tank” version or “Panzer III chassis?” Panzer III as a tank, was not the most produced German tank because of its size constraints. However, if you count the Panzer III chassis (StuG III), it was the most produced chassis.
At the beginning, Panzer II with 20mm gun was the mainstay. Panzer I only mounted machineguns. Both were quite easily destroyed.
Panzer III with 3.7cm gun was designed to be a tank killer.
In Africa, Panzer III even mounted a long barreled 5cm L60. The British called it ‘Panzer III Special.’
Panzer IV with short barreled 75mm L24 (below) was only supposed to be a bunker buster. Anti-tank projectiles are hard objects designed to puncture an armor plate. Bunker buster/anti-infantry projectiles are designed to inflict damage in a wider area by exploding. A fatter 75mm High-Explosive shell can pack more than twice the explosives than a 50mm shell.
As soon as Germans met T34, it seemed Panzer III as a tank became useless (but the chassis wasn’t). T-34 had a long barreled 76mm gun that could destroy everything Germans had.
German guns were ineffective, especially 37mm Pak 36 that were mounted on wheels or Panzer III. It could destroy T34 from the side at very close range like 100m. From the front, it was a “door knocker.”
5cm guns like Pak 38 below, or on up-gunned Panzer III, was a better tank killer than the 3.7cm gun. But it wasn’t effective against monsters like KV1 and KV2. (It could penetrate KV1, but only with a tungsten shell, within about 500m. But Panzer III would be destroyed long before getting that close.)
Clearly something more powerful was needed. But Panzer III’s turret ring (i.e. chassis width) was not large enough to mount a long barreled 75mm gun.
Because of the increased size of breech, they could only mount it to Panzer IV tank, which had a larger, wider chassis.
This meant Panzer III was useless…
Or was it?
Germans already used dozens of Sturmgeschütz (StuG) III during the invasion of France. It was General Erich von Manstein’s idea. Artillery is called to destroy bunkers, and he thought something like this was suitable ‘nutcracker’ than guns on wheels. It had thick frontal armor to go up to a fortified position and blow it up with a high explosive shell. General Heinz Guderian (later Chief of Staff for German Army) didn’t think it was necessary because Panzer IV mounted the same exact gun.
It uses Panzer III’s chassis. While it’s not a ‘tank’ per se, StuG was one of few vehicles heavily armored and low enough to sneak around T-34 and knock it out. When longer 75mm L43 was available, only Panzer IV and StuG IIIs could mount them. This gun could penetrate 82mm of 30 degree sloped armor at 1,000m. Long distance T-34 hunting became possible.
Below is a scale model, but you can see how long the 75mm gun is. Panzer IV could put 3 men and the gun breech in a turret. But for Panzer III chassis, this was the only way to make them fit.
It was cheap too. For the price of a Panther (175,000 RM), they could make 2 StuG IIIs (82,000 RM each) and some change left over.
Later, slightly more powerful (longer) 75mm L48 gun became the mainstay on StuG III, just like Panzer IV.
In the end, 8,500 Panzer IVs were built. 6,000 Panthers were built. Compared to that, 5,700 Panzer III wasn’t a lot.
But if you add 10,000 StuGs and 1,300 StuHs (105mm howitzer version), total of 17,000 Panzer III’s chassis would be the most produced tank-chassis for Germany. That’s more than Panzer IVs, Panthers and 1800 Tiger I & IIs combined. So… basically more than all other late war German tanks combined [excluding light tanks like Panzer I, II and 38(t)].