The witches establish their presence in the first scene, thus establishing a feeling of foreboding and evil. And there is a subtle indication that they might have some influence in the events to follow. The First Witch asks if they should meet, "In thunder, lightning, or in rain?" The Second Witch replies that they will meet when the battle is over. So the question was "when" will they meet. However, witches were thought to have the capability of affecting the weather, so one motif that is established is the witches' evil presence and this alludes to their potential influence.
The other motif, more directly stated, is the potential for circumstances (and people) to go from good to bad. "Fair is foul, and foul is fair," states this motif. As the plot progresses, Macbeth, the once good and loyal subject, will become evil.