If you are supposed to come up with exactly three key events or policies, you really should look at your textbook or your notes. This is because there are many key events and policies that led to the war and it is a matter of opinion as to which of these are the most important.
The key event that led directly to the outbreak of war in 1914 was the assassination of Archduke Franz Ferdinand of Austria-Hungary. His killing led almost directly to the outbreak of war.
With regard to more general policies, one can argue that the main cause of the war was the Germany policy (led by Kaiser Wilhelm) of enlarging its navy and trying to get its "place in the sun." These actions were perceived as aggressive, which led England and France to form alliances against Germany and to be much more ready to go to war against that country.
These are some of the key events and policies that led to the outbreak of war. Once again, I suggest that you should check your text or notes to see if they have a specific three events or policies that are deemed to be the most important.