The IS-3 came too late to see action in World War II. Though some older sources claim that the tank saw action at the end of the war in Europe, there are no official reports to confirm this. The tank saw no action against the Germans, although one regiment may have been deployed against the Japanese in Manchuria.
The first test group of JS-3's left the factory gates in mid-May 1945. Despite western opinion (usually I hear about JS-3's seen on the streets of Vienna), JS-3 tanks weren't involved in battles on the Eastern Front. The participation of JS-3's in Far Eastern Front battles (in August 1945) is still unverified: at least one tank regiment of JS-3 tanks was sent, but Soviet combat records don't confirm any actual combat.
You can be sure soviets hide/erase well those records and nonone knows about them nowadays. There is a book from a former soviet tank driver on WW2 that wrote he used a JS3 on the streets of Hungary (thanks to @neosdark).
Fascinating. I would like to see a movie / game about August storm. Would be cool ^^
It is a subject that has been discussed more than once, but without any conclusive evidence. They did not see combat in the European theatre, but they where according to some "deployed" at the victory-parade in Berlin. You would think, as I do, that the Soviet Union would take the opportunity to battle-test them in Manchuria, but I've never found any evidence, one way or another. So if anybody can come up with any conclusive information, I, too, would be happy.
On September 7, 1945, a united Allied military parade was held in Berlin. The parade began with infantry columns: Soviet soldiers of the 9th Rifle Corps of the 5th Shock Army, then soldiers of the French 2nd Infantry Division, Alpine troops, and Zoaves. There was a brilliant display by the British 131st Infantry Brigade. Paratroopers of the US 82nd Airborne Division brought up the rear. After a short break, a continuous mechanized column started.The first vehicles were 32 M24 «General Chaffe» tanks and sixteen M8 APC's from the US 705th Armored Battalion, and then tanks and armored cars from the French 1st Tank Division. After that, 24 «Comet» tanks and 30 armored cars from the British 7th Armored Division. And after that... rolled 52 of the newest JS-3's! That was a complete shock for all the Allies! A combined regiment of JS-3's was formed, based on the 71st Guards Heavy Tank Regiment (this unit is remembered as having fought with first King Tigers on the Eastern Front).