The IS-3 was another Russian heavy tank, this time with a spherical turret instead of the normal sloped turret. It was considered a wonder, with many, many false claims involving the United States and the ruler of the United Kingdom. It was developed in 1944, seeing a small amount of combat in WW2. It had 250mm armor, with a total of 400mm of effective thickness. It had a 122mm cannon, with 1 DT MG, and 1 DShK machine gun, being used as anti aircraft. It was a very good tank, being used in its time in WW2 and the Cold War.
The IS-3 was one of the first tanks to have a rounded turret instead of sloped, making the armor stronger than other kinds at the time. It was first presented at a parade in Moscow and that parade was one that spread misinformation about the U.S. and British reactions to the new tank. It had armor between 60 and 175mm armor depending on what you were looking at. The ammo was stored in the back turret, where there wasn't as much armor which would result in shells being able to go through the armor and explode the shells. When this happened the tank would usally be disabled or destroyed, however it wasn't seen in many wars, being produced just after WW2.
The IS-3 had a D-25T 122mm gun, being able to shoot through many different types of armor, and having the ability to fire many different kinds of shells. It could have one DT 7.62 coaxial machine gun, and another 12.7mm DShK machine gun being used for anti-air. It was able to mow through enemy infantry and shoot down low-flying aircraft with ease.
The flaws of the IS-3 were the hull welds snapping off due to stress, or that the main gun was so large and heavy compared to the small turret, which made the gun depression much worse than what it could've been. If the cannon breech is large, like the IS-3s gun needed, then the turret needs to be pretty big, otherwise the cannon breech wouldn't be able to go down all the way.
The engine was a Kharkiv V-2 engine, having 600 horsepower with 2000 rotations per minute, making the engine not that bad, however the transmission was ok, like most Russian tanks at the time. It had production problems that led the transmission to be worse than other tanks at the time.