From the article:
Unlike the Western nations, the Soviet Union continued with heavy tank development long after WWII. First seen by Western nations at the 1945 Victory Parade in Berlin, the IS-3 was an evolutionary development of the Iosif Stalin series of heavy tanks used extensively during World War Two.
The IS-3 heavy tank made excellent use of sloped armour on the hull and had a hemispherical turret which would be seen on nearly all post-war Soviet tanks. Armed with the outstanding D-25T 122mm gun, the IS-3 was, however, relatively slow and suffered from reliability issues, especially in the hot desert.
Egypt acquired roughly 100 of these tanks, assigning them to support the T-34/85 tanks in the infantry divisions and back up the T-55 tanks in the mechanised and armoured divisions. Their combat results during the Six-Day War were not auspicious.