Taigen V2 gearboxes. They have bearings on every shaft and I use them in all of my tanks. For the pz iii you'll want the 3:1 mid lows with 48mm shaft, if I recall correctly.

My only issue with them is they can be loud, but I'm working on that. I noticed that in my metal hull Sherman, that the steel gearboxes are very quiet. I'm thinking that they are loud in my plastic hulls because they can vibrate.

Those are some sharp looking tanks BTW.