There is a close relationship between plot and setting in act 1. What are three examples that reflect the decline into evil?

The most obvious element of setting which sets the scene for the evil that follows is the rain and thunder which accompanies the witches' appearance and predictions.  Soon after we meet them and they tell of their intention to see Macbeth later, Macbeth himself comments that he has not seen so foul and fair a day before in his life--the weather has an effect on him, yet it has not kept him from winning the battle and the titles to come.

After Macbeth's meeting with the witches, Macbeth writes a letter to inform his wife of their prophecy to become Cawdor and King.  Within minutes of their prophecy, Ross rides in to bestow upon Macbeth the title of Cawdor.

Before Macbeth even gets home, his wife is planning Duncan's death.

Shakespeare often uses weather and animal imagery to show us the inner feelings and conflicts of his characters.  The rain, thunder, and the unsettled events of animals and weather on the night of Duncan's murder tell us that God, the Earth, and all of nature disapprove of Macbeth's decision and actions.

This weather sets the tone for the evil in all of Act 1 which continues through the entire play.