A good place to look for lines which support Macbeth's being a victim of the three witches is Act 3 Scene 5. In this scene, Hecate scolds the witches for telling Macbeth his fate. Hecate calls Macbeth a "wayward son" which suggests that he is too easily swayed by his own ambition and greed to make the right decisions regarding his future. Hecate tells the witches that they have been "saucy and overbold" in their dealings with Macbeth and that they must correct the situation. They all know that Macbeth plans to visit the witches for another foretelling, and Hecate commands the witches to cast a spell. Since Hecate is scolding the witches for their dealings with Macbeth, the reader assumes that Macbeth is their victim.