The three parts of The Reader are connected at first sight only by the chronological course of the story. Because of the large leaps in time, there are only small emotional connections between the parts. The transitions are all abrupt to show the reader the change in the following chapters. However, the contents of the different parts fit together and you can see various stages of development, for example, the relationship between Hanna and Michael, or even in the breeding of Michael. These stages build on each other as logical consequence, but they are not directly connected in the book.
In conclusion, you can say that the three parts of the novel are connected by their meaning and they are therefore not interchangeable. Their content is separated to make the progress and development stages particularly clear. There are also recurring motifs that occur in all parts. For example, the association of Hanna with a horse. In the first part, Michael calls her “horse” as a nickname, and in the second part, the reader learns that the concentration camp inmates called her “Mare”.