I think the three events that Hannah might remember are,
-In chapter two Hannah watches her grandfather yell at the t.v. screen. She sees his a five-digit number tattooed on his arm. Although she doesn't understand the full meaning of it then, I think this memory will haunt her as the story progresses and even after it concludes.
-In Chapter 8 Shmuel is about to be married and everyone is so festive. The Nazis show up and demand that everyone must get in the trucks for "resettlement". This is the first intrusive overtly disturbing stations Hannah has with the Nazis. There is always a sense of dramatic irony for Hannah because she knows what happens to Jews.
-Hannah gets her own number tattoo. This ties in with number 1 as well as a literal and symbolic representation of Hell.