One of the most significant events to take place in chapter 3 of Wiesel's Night would be the initial selection that he experiences at Birkenau. This was a selection in which men and boys were moved to one side while women and girls were moved to another. It is a significant moment for a couple of reasons. The first is that Wiesel speaks of it as the last time he sees his mother and his little sister, Tzipora. Another reason why it is significant is that it shows the complete random condition of life and death in the Holocaust. Eliezer does not know what to do and in the midst of emotional and physical chaos, someone yells at him to lie about his age and his father's age. Eliezer does not know if the person who hisses the words of advice to he and his father is trying to save them or condemn them.
Another significant event of chapter three is the mere image of death that the Holocaust represents. It is this moment in the narrative that Wiesel renders his "Never Shall I Forget" poem and it is significant because of its mournful tone. It is also significant as it expresses the political and personal violation of rights that the Nazis perpetrated upon millions of human beings. The description of death that is rendered in chapter three is the first introduction to an infernal state where dehumanization results in the most heinous and horrific of criminal activity.
Finally, I would suggest that another important event of chapter three is that Eliezer and his father live past the crematorium and begin their work service in Buna. This will form the remainder of the narrative. It is in this where the physical and emotional degradation of the Nazis and others continue. It is also where Eliezer will begin to undergo significant transformation as he uses the moment of his initial selection as the breaking point between he and his religious faith and spiritual identity.