Hi Aydin: Well you are right the soviet zis 3 gun is not a good AT gun. Basically this weapon in real life was an artillery gun and soviets use it with AT purposes. Basically his low velocity gives the gun a poor AT capabilities.
Here some stats comparatives between pak 40 and zis 3 gun:
Penetration at 100m/500m/1000m
Pak 40:
PzGr 39 APCBC-HE: 116mm/105mm/93mm
PzGr 40 APCR: 157mm/132mm/106mm
Zis 3 76.2:
BR 350A AP-HE: 82mm/75mm/67
BR 350P APCR: 119mm/89mm/60mm