If you're after certain structural elements instead of simple partitions of the story, consider these -
Use of the third person limited point of view - The story takes place from the viewpoint and understanding of the protagonist only. Most of what happens is only in his mind, as the reader learns at the end of the story.
Use of flashback and virtual reality - Although the reader is not aware of it at first, the "events" in the story don't happen in chronological order but are subjectively selected (between real and imaginary ones) by the character of Farquhar.
Use of dramatic irony and a surprise ending - Up until the very end of the story, the reader "lives" the events of Farquhar's fantastic escape. Just as he "makes it," though, the reader realizes it has all been an illusion as Farquhar's neck snaps under the tension of the rope.
Your question is a bit unclear, but I assume you are referring to the main elements of the three parts of the Ambrose Bierce short story, "An Occurrence at Owl Creek Bridge."
In Part I, Peyton Farquar is about to be hanged on the railroad bridge over Owl Creek. He sees everything more clearly and in slow motion as his execution nears. As he thinks about his home and family, the captain signals the execution to be carried out.
Part II is exposition, a flashback that recounts Farquar's past and the actions that have put him in the hangman's noose. He has planned to burn the Owl Creek Bridge, but he confides in a Southern soldier who is secretly a Northern scout--a spy.
Farquar escapes the noose at the beginning of Part III: The rope has snapped and he tumbles into the water below. He struggles to break free of his restraints and rises to the surface, only to be met by bullets and cannon fire. But he escapes, swims to safety and heads back home to see his wife and kids. At last, he sees his house and reaches out to hug his wife... Or does he?
The final sentence explains the truth:
Peyton Fahrquhar was dead; his body, with a broken neck, swung gently from side to side beneath the timbers of the Owl Creek bridge.
The use of deception--both on the part of Farquar and the Northern spy as well as the author's creative way of misleading the reader--is a major theme. Farquar's manipulation of time is another focus of the story. The relationship of death vs. reality is yet another important motif that makes this one of the finest of all American short stories.