I find it hard to believed that the PZKW III could be developed beyond the potential of the PZKW IV.
The Chassie of the Pzkw III was very useful in assult guns that packed more firepower then the tank. But by 1944 any development from the III would have been close to obsolete. As they found the with the IV. If they put effort into sabot rounds or taperbore cannon for the 50mm they might have squeezied another year of frontline service out of the III. But taperbore has terrible wear factors, and tungston was in short supply at times so sabot rounds would have been costly. If they had brought out PZKW V without the teething problems it would have been a mute point anyway.
The Chassie of the Pzkw III was very useful in assult guns that packed more firepower then the tank. But by 1944 any development from the III would have been close to obsolete. As they found the with the IV. If they put effort into sabot rounds or taperbore cannon for the 50mm they might have squeezied another year of frontline service out of the III. But taperbore has terrible wear factors, and tungston was in short supply at times so sabot rounds would have been costly. If they had brought out PZKW V without the teething problems it would have been a mute point anyway.