I use this often...
Send in any infantry near the tank to scare it and make it try and attack your men at close range...
Get the IS3 to turn 180 degrees from your opposing tank (waiting to fire)... if the infantry distract it well enough.. you can get some great hits (and even charge the IS with your AT/Tanks) and pierce the rear turret which seems to be much easier to take out. Use your machine guns to damage the treads between AT rounds... If your infantry have AT grenades then this will really help... but rear turret shots are pretty good enough to damage it... If using direct control... aim at the very bottom of the turret right near the gap to the body base... Like opening a can of tuna!