Please stop spamming suggestions like this. Before you post, think of why you want your suggestion to be implemented. And justify why.
You can't compare v1 to IS3, because V1 was made by Relic and is no tank that you can produce. It's an ability. Correct would be:
"My suggestion is IS3 because it was vastly (what anyway is not true, just as an exapmle) used on eastern front in late war and it would be a counter vs. King Tiger or other heavy German battle tanks."
Anyway, Soviets don't need IS3 cause they are OP even without it and they have IS2.