You could write your paper from the perspective of technology's role in the development of the U.S.  If you went that route, I would suggest: 1) factory system (Industrial Revolution), 2) the steam engine (led to Western Expansion), 3) the atomic bomb (established U.S. as super power, ended WWII).  It's incredibly difficult to pick just three, but I recommend that you really just choose and focus on the three you feel the most comfortable writing about, where you can provide the most detail.

Do you need speciffic events or can you use broader trends as your turning points? You have a number of events that could be classed into one general category as "industrial revolution"; another broad area of change could be "civil rights".

A third area that no one has mentioned might be "medical advances" - as health care has improved, more people are living longer, which is going to have an increasing impact upon health care costs, consumption of resources, decisions regarding new manufacturing, and in almost all other facets of society.

Many people who have been around for a while consider the John F. Kennedy assassination to be a turning point because it led to a "loss of innocence" on the part of the American people. It was immediately followed by the most explosive years of the Civil Rights struggle and the Vietnam War. For these reasons, I think the 60's were, at least up until now, the turning point in American history. It isn't easy for a nation to change, but we changed in that decade.

I think the ratification of the Constitution, the end of the Civil War, and the Great Depression were the key turning points in American history, if I had to choose only three. The ratification of the Constitution obviously set the nation in motion. The Civil War led to the end of slavery and marked an end to the sectional debates of the nineteenth century--for the most part. The Great Depression created the climate for the New Deal, which was a remarkable change in the relationship between Americans and their government.

I agree with above posts in regard to the Cotton Gin/Civil War point and the industrial revolution, and I would add our defining win of WWII which cemented our place as a the most powerful nation in the world. All of these events shaped the economic and political place that makes America a great place to live.

Here's my three major events that were turning points in American history:  The Revolutionary War, World War II, and the atomic bomb.  The Revolutionary War was a turning point because if we hadn't fought it, we might still be part of Great Britain and would never have become an independent and free nation. World War II was a turning point because if we hadn't fought it and won, we might all be speaking German or Japanese. Those two nations were cruel, ruthless, and determined to overrun the whole world. We had to fight them to protect our nation, our way of life, our freedoms. The creation of the atomic bomb was a turning point because a whole new power was discovered that when unleashed, could destroy everything in its path. The advent of the nuclear age has ushered in a whole new era and we'll never be the same again.

A good book to reference is  Turning Points: Making Decisions in American History by David Burner and Anthony Marcus.

I agree with the previous post that the invention of the cotton gin would be one of them because it helped bring about the Civil War.  For a third, I would pick something like the 1960s because it changed the entire society and culture in which we live.