Final answer:
At the Yalta Conference, the 'big three' were Roosevelt, Churchill, and Stalin. By the Potsdam Conference, Roosevelt had been succeeded by Truman, and Churchill had been replaced by Atlee after the UK elections. They discussed post-war Europe and the defeat of Germany.
Step-by-step explanation:
The 'big three' Allied leaders at the Yalta Conference in February 1945 were President Franklin Delano Roosevelt of the United States, Prime Minister Winston Churchill of the United Kingdom, and Soviet Premier Joseph Stalin. Together, they discussed the post-war reorganization of Europe, including the demand for Germany's unconditional surrender and the proposed division of Germany and Eastern Europe into spheres of influence. These plans also included preparations for the United Nations and considerations for the ongoing war against Japan.
At the subsequent Potsdam Conference in July 1945, the Allied leaders changed. With Roosevelt having passed away in April, he was replaced by President Harry S. Truman. Churchill was replaced partway through the conference by new British Prime Minister Clement Atlee, following the Labour Party's victory in the UK general elections. Stalin, Truman, and Atlee finalized the plans for postwar Germany, which included its division into occupation zones.