I can imagine the M36 replacing the M26 Pershing but that would kinda be a waste of time, since the M36 was much faster, had the same gun but very little armor, making it easy pickings for even a Panzer IV if used properly. The M26 if it ever gets a reward unit, should be replaced with either the Super-Pershing with the extra armor and slower speed, make it slightly more similar to a King Tiger, only 1 call-in per game or the Jumbo Sherman(which should be 2 or 3 on the field at once), which has more armor, and a smaller gun.
And yes the IS-3 was by the end of WWII in deployment with several units, but no concrete evidence exists about whether they fought in Europe or not. I'm pretty sure that a few were used in August Storm, but other than that, no one knows.