At the beginning of act 1, scene 3 of Macbeth, the "Weird Sisters" (the three witches) are gathering to meet Macbeth, which is something they arranged in the first scene of the play:
FIRST WITCH: When shall we three meet again?
In thunder, lightning, or in rain?
SECOND WITCH: When the hurlyburly's done;
When the battle's lost and won.
THIRD WITCH: That will be ere the set of sun.
FIRST WITCH: Where the place?
SECOND WITCH: Upon the heath.
THIRD WITCH: There to meet with Macbeth. (1.1.1–8)
The witches are already "on the heath" in scene 3 when Macbeth and Banquo show up. The witches have been standing around talking, basically killing time until Macbeth shows up, and one witch shows the others a thumb that she got from a sailor who lost it in a shipwreck; they all seem pretty excited about the sailor's thumb.
When Macbeth and Banquo arrive, they're a little taken aback when they see the witches.
BANQUO: . . . What are these
So wither'd, and so wild in their attire,
That look not like the inhabitants o' the earth . . .
You should be women,
And yet your beards forbid me to interpret
That you are so.
MACBETH: Speak, if you can. What are you? (1.3.40-49)
The Witches ignore Macbeth's question:
FIRST WITCH: All hail, Macbeth! hail to thee, Thane of Glamis! (1.3.50)
This is not a prediction. Macbeth is already Thane of Glamis.
In Scotland in the eleventh century, the time period in which Macbeth is set, a "Thane" was a nobleman who was permitted by the King to own land in exchange for his military service to the King. (By the way, "Glamis" is correctly pronounced "Glahms," in one syllable.) Macbeth has distinguished himself on the battlefield, and is well known to King Duncan, which gives rise to his ascension to this title.
The second witch speaks up:
SECOND WITCH: All hail, Macbeth! hail to thee, Thane of Cawdor! (1.3.51–52)
Macbeth is confused, since he's not "Thane of Cawdor":
MACBETH: . . . By Sinel's death I know I am Thane of Glamis;
But how of Cawdor? The Thane of Cawdor lives,
A prosperous gentleman (1.3.74–76)
This looks like a prediction, but it's not.
In scene 2, we learned that the Thane of Cawdor was a traitor to the King in battle, and the King was seriously annoyed about that.
DUNCAN: No more that Thane of Cawdor shall deceive
Our bosom interest. Go pronounce his present death,
And with his former title greet Macbeth. (1.3.74–76)
Angus and Ross are going to show up a little later in scene 3 to tell Macbeth that he's now Thane of Cawdor. So even though this is news to Macbeth, it's not a prediction.
Once again, the witches ignore Macbeth's question, and finally get to the point:
THIRD WITCH: All hail, Macbeth, that shalt be King hereafter! (1.3.53)
This is the prediction we're looking for: that Macbeth is going to be King. This might sound like a good thing, except that it sets in motion Macbeth's and Lady Macbeth's extreme ambition, their ruthless, murderous path to the throne, Lady Macbeth's suicide, and Macbeth's death at the hands of Macduff.