What language techniques, such as language and poetic devices, does Shakespeare use to present the Witches in Macbeth, Act I, scene iii?
The Weird Sisters first speeches rely heavily upon symbolism to introduce their natures. For example, the First Witch says, when asked where she has been, that "A sailor's wife had chestnuts in her lap." Chestnuts are a symbol of presentiment, or foresight, and prevention. Someone standing near said to the Witch that the woman's husband had just sailed to Alepo. Since the woman "munch'd, and munch'd, and munch'd:--" the sweet chestnuts, we can understand this as an effort on her part to forefend any harm befalling him on his journey so that he might come home safely to her. In other words, she is practicing a bit of superstitious magic of her own on her husband's behalf. This would explain the Witch's anger at the wife's refusal to give over the chestnuts when the Witch demanded them: the Witch seemingly objected to someone else having foresight.
The Witch responds that she will be like a rat (without a tail), symbolic of greed for gain, and that she will take all he has:
I will drain him dry as hay:
Sleep shall neither night nor day
Hang upon his ... lid;
She says he shall "dwindle, peak and pine" because for seven nights multiplied nine times nine, he shall be weary and sleepless: "Weary se'nnights nine times nine." Nine symbolizes the power of completion and attainment of objectives. When used by the wrong hands, like those of the First Witch, it is manipulative and may cause destruction to others as the users objectives are attained.
Thus one language device Shakespeare uses is describing the Witches through their inner motives instead of through their actions or physical characteristics. A poetic device he uses is symbolism, using complex symbols (food, animals, and numbers). It is possible that these symbols were as commonplace to everyday living situations then as allusions to baseball scores are commonplace today. It's possible these symbols seem complex to us whereas they might have seemed quite ordinary to Shakespeare's audiences.