what instances of flashback and flash forward can you find in parts two and three of the book thief? what do you learn from these interruptions to the normal sequence of the plot?

Oh I see what you mean by parts, sorry.

We are able to learn a lot about characters backgrounds, Hans in particular. Here is an excerpt from GradeSaver part 2,

Hans' character becomes more complex as it is revealed that he is not a member of the Nazi Party, his application having been tabled because of his willingness to paint Jewish homes and cover up Jewish graffiti. As a result, Hans' business suffers despite his talent, his standing in the community becomes threatened despite his affability, and his relationship with his son is ruined despite their former closeness. Hans Junior is fully indoctrinated in Nazi ideology and speaks in broad metaphoric terms of restoring Germany's greatness by eliminating social undesirables -- unbeknownst to him, this would include Liesel. He revolts against his father and says that Hans is part of an "old, decrepit Germany," one which lost World War I and is supposedly being rebuilt and avenged by the Nazis. Ironically, Hans Junior calls his father a coward for privately opposing Hitler and the Nazi platform, yet Hans' willingness to help Jews is much more dangerous than supporting Hitler. The meaning of cowardice and the question of whether or not Hans is a coward is a moral question that is revisited throughout the rest of the novel.

As you can see Hans's character is fleshed out much more by going back in time. The same goes for part three where characters like the mayor's wife, and Max Vandenburg are illuminated.