What were the three main elements of "An Occurrence at Owl Creek Bridge"?

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.