The fact that they are witches automatically implies evil and wickedness so their plan to meet with Macbeth suggests that they and perhaps Macbeth himself are up to no good. When they do greet him they greet him by two titles we know he has: Thane of Glamis and Thane of Cawdor. The latter, however, he knows nothing about so when Ross comes to tell him that the King has bestowed this title on him as a reward for his valour on the battlefield, Macbeth is mystified. 'The thane of Cawdor lives/Why do dress me in borrowed robes?' (Act 1:Sc 3 L107-8) It is then he learns that the man who had that title had turned traitor and awaits execution. To him it seems the witches speak truly, to an audience, we know it was Duncan's gift to Macbeth for services rendered.
This is the scene that reveals Macbeth's ambitious desires. From the moment the witches hail him as 'King hereafter' he reveals his 'black and desires' (Act 1:Sc 4 L51) In response to his new title he muses to himself whether or not what the witches have foretold is good or bad, coming to the thought If good, why do I yield to that suggestion Whose horrid image doth unfix my hair, And make my seated heart knock at my ribs Against the use of nature? (Act 1: Sc3 L133-136)
Both excited and alarmed by what he is thinking 'my thought, whose murder yet is but fantastical' he is nevertheless shaken by the direction his thoughts are taking. Banquo's notices how 'rapt' Macbeth is when first he heard the prophecies and now, on hearing Ross' news he notes once again the spellbound state Macbeth seems to be in. These reactions visible to Banquo who heard the witches' prophecies, are further evidence that not only was Macbeth ambitious, he had already