Okay, this is similar to Offseason Timewaster #1.
The only change is that now you can participate in the historical event to change history.
Again, pick three that you would go back to, and tell us what you'd do to change history.
My three are:
Sometime in the early 1930s, I'd be in Germany and I'd assassinate Adolf Hitler. Would the Nazis have still come to power, under Goering, Goebbels, Heydrich, or some such? And would World War Two and the Holocaust have necessarily followed?
14 April 1865 would find me in Ford's Theater, and I'd stop John Wilkes Booth from shooting Abraham Lincoln. How would things have turned out had Lincoln lived? One thing I expect would change, I doubt that Reconstruction would have been the mess it turned out to be had Lincoln still been in charge.
22 November 1963, I'd be on the 6th floor of the Texas School Book Depository, and at the appropriate time I'd nudge Oswald throwing off his aim (or, for the conspiracy minded, I'd be on the Grassy Knoll or wherever and knock the mystery shooter's aim off) so that Kennedy lived. How would US history have turned out had he lived and run for a second term?