I will name three important events, but I do not know if they are the most important.
The American War of Independence set us up and got us going.
Considering the amount of resources and territory that were added to the United States, the War with Mexico was a very important event.
Ditto the Louisiana Purchase.
Considering what resulted, one very important event was the election of Abraham Lincoln as President.
Well, that is four.
Important because of their impact are Abraham Lincoln, Woodrow Wilson, Franklin Rooseveldt.
Three that I am able to admire are George Washington, John C. Breckinridge, Robert E. Lee.
The previous thoughts are exactly right. Without criteria, this becomes intensely difficult. I am going to suggest that my criteria for the most important people/ events would be the voices of dissent. I think that America is one of the few nations that has been constantly defined by individuals who voiced dissent against what was in the hopes of articulating a vision of what should be. In my mind, these voices of dissent helped to form the nation and represent a benchmark of what America has always been and, perhaps, what it should be. Thomas Paine and "Common Sense" would be one such example. The voice of dissent inspired Americans to form a new nation, develop a new voice, and seek to create something from nothing. Another such example of dissent would be Frederick Douglass and the voice of the abolitionist. Douglass helped to bring home the idea that the promises of America cannot coexist with its reality of slavery. America, in Douglass' mind, was a walking contradiction between what should be and what is, which is far from it. I would finally say that Emma Goldman might be a great example of a modern voice of dissent. In a post Civil War America that embraced modernity, industry, and economic notions of the good, Goldman brings out the idea that there is another side to such a vision. There is another voice that is being crushed under the heel of so- called progress. Her speeches call to light that the Status Quo concept of consensus is not applicable to those who experience something else, and this becomes the hallmark of the voice that we hear and need to hear more of today. It is the voice of dissent that allows America to become more than what it is, providing better tomorrows to yesterdays that were not entirely bad with which to begin.
That's hard to say without criteria as to what's important, but if I had to choose, I would rank my events like this, assuming American History starts at settlement:
1) Ratification of the Constitution - more important than the revolution itself, as those are common. Replacing them with a democracy is not.
2) The Civil War - because this conflict could have destroyed us as a unified nation, and our history afterwards would have been much, much different. The war also made it possible to free the slaves permanently by law.
3) World War II - Saved us from the Great Depression, unified the country and established the US as a superpower with nuclear weapons.
My three most important people would be:
1) George Washington - tireless public servant who refused to take advantage of us and make himself King.
2) Abraham Lincoln - guided us by whatever means possible through the Civil War, put his own personal feelings aside and did what he thought best to unify us. Emancipated the slaves. Brilliant writer and speaker.
3) Franklin Delano Roosevelt - Guided us through two national crises - the Great Depression and World War II. Served his entire adult life.
I know those are all Presidents, but you only let me choose three.
According to me the three most important event of American History - and by this I mean History of United States of America - are discovery of American Continent by Columbus, American War of Independence, and American Civil War. The discovery of American continents by Columbus led to the knowledge of these place among Europeans and people from other part of the country, which ultimately led to immigration of people in large numbers. Without such Immigration, no new big countries, including the USA would have been formed there.
American war of independence is definitely the most important event in history of the USA as it resulted in birth of the country. Without this there would have been no country called America, and there would have been no history of America. The American war of independence, in being the most important Event for America, it has also made a deep and lasting impact on the world history by inspiring establishing of democratic countries across the world.
The American civil war is important for two reason. First is the obvious reason of establishing USA as a country which not only talks about equality and freedom of people, but also strives to achieve this in reality. I am sure this event has a major impact on forming the basic character of American people. Another major contribution of the civil war was to tighten the bond between the states within the nation. Defeat of the separatists in the civil war made the USA more closely united for all times to come.
Talking of three most important people in American history, I would put Benjamin Franklin at number one. I am a great fan of Benjamin Franklin, and believe that he played a major role behind the scenes in bringing people together in formation of the USA and in deciding its character. At number two comes Abraham Lincoln. I will place George Washington at number three for the leadership he provided during the war of independence.