The three witches fulfil a vital role in Macbeth as they represent all the deepest fears that the Elizabethan audience may have had regarding evil, spirits and the supernatural.
In Shakespeare’s day, evil was believed to manifest itself in reality and, whilst apparitions and hallucinations formed part of the existence of the supernatural, there was a concrete element, and the witches therefore reinforce the reality of it. The fact that Macbeth begins with the witches and the "fair is foul and foul is fair" (I.i.10) provides the audience with the tangible proof they need. The witches are “not like th’ inhabitants o’ th’ earth, / And yet are on’t” (I.iii.41)
The audience is thus well prepared for the apparitions in ActIV.i. and, having been introduced to the witches earlier, the sense of foreboding is entrenched in the superstitious audience who must have been captivated.
The first apparition is "an armed head" which is a fitting illusion as it represents the forthcoming battle and confirms what Macbeth had thought - "Beware MacDuff."(IV.i.71)The audience can see Macbeth building up to something as he asks for more.
The second apparition - "A bloody child"- tells Macbeth that "none of woman born" (80) shall harm him. The child represents the threat of MacDuff and in fact Banquo's children and in fact ANY child born. Macbeth is in a stupor so cannot rationalize that this must have a deeper meaning. The audience will also be suitably enthralled so will not makes sense of the comment nor seek to analyse it further; save to know that each apparition is motivating Macbeth more and more.
The third apparition confuses Macbeth at first as he feels he has basically already defeated all humankind through the words of the second apparition. The third apparition is a "Child Crowned, with a tree in his hand" but instead of realizing that this in itself is a warning that there is more than meets the eye and "fair is foul and foul is fair" as the baby apparition wears "the top of sovereignty (88), Macbeth is encouraged by the fact that Birnam Wood can surely not "come against him."(93)
The audience can now see Macbeth falling into a trap. The audience too is confused but as it will have been superstitious itself, none of the people would have wanted to admit their confusion; they would not however have doubted the efficacy of the apparition and it's warning - foreboding - of imminent trouble.
A "show" then appears as Macbeth is not satisfied by the apparitions. The witches let Macbeth do all the talking as it is his imagination that gives credence to everything he has seen - or imagined!