In all three cases, these battles turned the tide of war. The Axis had been on the offensive since the beginning of the war in all theaters of the war. With these three battles, the tide turned and the Axis was no longer able to advance. From then on, they were on the defensive.
After Midway, the US started to move from island to island across the Pacific, moving closer to Japan. After El Alamein, the Germans were no longer able to threaten Egypt and the Suez. After Stalingrad, the USSR was no longer in danger of falling.
The Battle of Midway, which resulted in the loss of four Japanese Aircraft Carriers, was the first major defeat for the Empire of Japan, and was the farthest their forces would advance in the Pacific. From 1942 on, there would be campaign after campaign aimed at "island hopping" back towards to the Japanese home islands.
At El Alamein in North Africa in 1942, British General Berland Law Montgomery ended any last chance the German Afrika Korps under Irwin Rommel had of taking control of Egypt and the Suez Canal. He then had to stage a 1500 mile retreat that ended with his leaving North Africa permanently the following year.
Stalingrad was perhaps the most significant of these three battles in terms of its affect on the war's outcome. This battle was massive in terms of the size of the German and Soviet Armies involved, the amount of material needed, and the obscene losses suffered by both sides. After five months of frozen, bloody hell, the entire German Sixth Army under Field Marshall Friedrich Von Paulus surrendered. 115,000 German soldiers were marched off to Soviet POW camps. Only 5000 ever returned home to Germany, and it was the last major offensive they could make on the Eastern Front.