When the Third Witch prophesizes to Macbeth, "All hail, Macbeth, that shalt be King hereafter!" (1.3.53), Macbeth has already been thinking about being King.
MACBETH. (Aside) Two truths are told,
As happy prologues to the swelling act
Of the imperial theme! ...
[W]hy 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? (1.3.138-140, 145-148)
It's only natural for someone in Macbeth's position as a great and renowned Scottish general to think about being King. (Julius Caesar followed this same path to become Emperor, and ultimately Dictator, of Rome.)
Shakespeare's Macbeth, like the real Macbeth, was a cousin to King Duncan, who refers to Macbeth as "valiant" and "worthiest cousin." Macbeth refers to Duncan's sons, Malcolm and Donalbain, as "our bloody cousins" when they flee to England and Ireland after Macbeth murders Duncan.
The real Macbeth, as Shakespeare would have known from reading Holinshed's Chronicles (on which Shakespeare likely based Macbeth), was denied being crowned King because Macbeth was thought to be "somewhat cruel of nature," whereas Duncan was considered by the people to be "soft and gentle of nature." (Holinshed's Chronicles, Volume V: Scotland, 264-265)
Being King would have been at the back of Macbeth's mind ever since he was passed over for Duncan.
The Third Witch's prophecy doesn't tempt or seduce Macbeth into thinking in the abstract about becoming King at some time in the future but tempts Macbeth into thinking about being King now.
In act 4, scene 1, the "apparition scene" of Macbeth, the First Apparition, "an Armed Head," warns Macbeth about Macduff.
FIRST APPARITION. ...Beware
Beware the Thane of Fife. (4.1.78-80)
This might well be a reflection of Macbeth's own thoughts and fears.
MACBETH. ...Thou hast harp'd my fear aright. (4.1.82)
It seems unlikely, however, that the two apparitions that follow are a reflection of Macbeth's own mind.
The Second Apparition, "a Bloody Child," tells Macbeth that "none of woman born / Shall harm Macbeth." The Third Apparition "a Child Crowned, with a tree in his hand," assures Macbeth that he "shall never vanquish’d be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him."
Macbeth seems truly surprised by these assurances, and rather than intensifying his fears, it instills him with a sense of invulnerability.
MACBETH. Then live, Macduff. What need I fear of thee?...
That will never be.
Who can impress the forest, bid the tree
Unfix his earth-bound root? (4.1.92, 106-108)
However, the apparition of the successions of kings, ending with the "blood-bolter'd Banquo" smiling at him and pointing at the succession of kings as Banquo's own doing, brings Macbeth back to reality and reawakens Macbeth's greatest fears.
MACBETH. Our fears in Banquo
Stick deep ... There is none but he
Whose being I do fear... (3.1.53-54, 58-59)