In the context of World War II, the 'Big Three' consisted of the leaders of the three most powerful countries on the side of the Allies. They were Winston Churchill, the Prime Minister of Great Britain, Franklin D. Roosevelt, the President of the United States, and Joseph Stalin, the Premier of the Soviet Union.
https://study.com/academy/answer/who-were-the-big-3-in-world-war-ii.html
Let me handle your first question -- always good to do one question at a time here. :-)
Prior to President Theodore Roosevelt, those who preceded him in federal government had tended to side with industry leaders, expecting laborers to fall in line and do the work for the good of the companies. In 1902, when there was a particularly tense strike by coal workers, Roosevelt invited both sides (labor leaders and management leaders) to the White House to negotiate. This was an example of the way he saw the role of government leadership as "steward" to the nation, mediating on behalf of everyone's interests, not just the interests of a powerful small group. His "Square Deal" policies were aimed at making things fair and square for the general public. An example of this would be how much land was set aside under his administration as national forests, national parks, national monuments, etc. He was seeking to protect the use of the land for all Americans' interests, rather than letting corporations tear into any land or forest they wanted in order to grab natural resources.
John Quincy Adams is the answer
