There is no doubt that they are real, for they manifest themselves in front of Macbeth and Banquo, but Banquo has doubts, for he asks Macbeth in Act I, scene iii,
Were such things here as we do speak about?
Or have we eaten on the insane root
That takes the reason prisoner?
It appears that Banquo is not sure that these witches are real for they vanish "Into the air" (I. iii. 81) like "bubbles" (I. iii. 78).
But make not mistake about it; they are real, for earlier in the scene when Macbeth and Banquo first meet them, Banquo gives us some interesting evidence to suggest that they are corporal; first, he makes mention of their "choppy finger[s]... / upon [their] skinny lips, and then he says that they seem like women, but because they have beards, he thinks otherwise. These images clearly suggest that the witches are real.
However, if Macbeth was the only one to see them, then we could argue that they are hallucinations; we know that Macbeth has visions, perhaps he suffers from sschizophrenia, but the fact that Banquo sees them makes them more corporal than hallucinations.
No, the witches in Macbeth are no hallucination. Banquo first notices the figures on the heath, and then Macbeth sees them too. We also see the three witches as Banquo and Macbeth see them and talk to them. Banquo categorically refers to some of their physical features like 'choppy fingers' and skinny lips', and wonders if they are women at all, for they grow beards. We also hear the conversation between the witches and Macbeth & Banquo.
Furthermore, the witches re-appear on subsequent occasions to enhance the plot. We see them along with their queen, Hecate; we see them as performing their rites in the cavern where Macbeth goes to meet them; they conjure up three apparitions to answer Macbeth's queries about future. The air-drawn dagger that shows Macbeth the way to Duncan's chamber is hallucinatory; so is Banquo's ghost at the Banquet, seen by Macbeth alone. But the elaborately presented witches are very much physical and objective.