By the end of scene 3, what has Macbeth revealed himself to be like?

We learned in act 1, scene 2 of Shakespeare's Macbeth that Macbeth is a brave and apparently ruthless warrior.

SERGEANT: . . . For brave Macbeth—well he deserves that name—Disdaining fortune, with his brandish'd steel,Which smoked with bloody execution,Like valor's minion carved out his passageTill he faced the slave,Which ne'er shook hands, nor bade farewell to him,Till he unseam'd him from the nave to the chaps,And fix'd his head upon our battlements. [1.2.18–⁠25]

Clearly, Macbeth is a man of action, and he accomplishes what he sets out to do, no matter the opposition or the consequences.
We meet Macbeth in person in scene 3, when he is on his way to King Duncan's castle at Forres with his friend and fellow warrior Banquo. Macbeth makes some remarks about the weather, and Banquo wants to know if they're there yet.

MACBETH: So foul and fair a day I have not seen.
BANQUO: How far is't call'd to Forres? [1.3.39–40]

The witches appear, and neither Macbeth nor Banquo seems particularly concerned about them appearing from nowhere, other than their looks.

BANQUO: What are theseSo wither'd, and so wild in their attire,That look not like the inhabitants o’ the earth,And yet are on't? [1.3.40–43]

The witches address Macbeth:

FIRST WITCH: All hail, Macbeth! hail to thee, Thane of Glamis!
SECOND WITCH: All hail, Macbeth! hail to thee, Thane ofCawdor!
THIRD WITCH: All hail, Macbeth, that shalt be King hereafter! [1.3.50–53]

Banquo notices that Macbeth is somewhat taken aback by what the witches tell him.

BANQUO: Good sir, why do you start, and seem to fearThings that do sound so fair? [1.3.54–55]

It might be that Macbeth is surprised to hear the witch say out loud something that he's already been thinking about—that he could be king. Macbeth thinks about it for a minute, then interrupts the witches' small talk with Banquo to ask them how they know what they know and why they're telling them this.

MACBETH: Stay, you imperfect speakers, tell me more.By Sinel's death I know I am Thane of Glamis;But how of Cawdor? The Thane of Cawdor lives,A prosperous gentleman; and to be KingStands not within the prospect of belief,No more than to be Cawdor. Say from whenceYou owe this strange intelligence, or whyUpon this blasted heath you stop our wayWith such prophetic greeting? Speak, I charge you.
[Witches vanish.] [1.3.73–81]

Neither Macbeth nor Banquo is particularly surprised or concerned that the witches just vanished into thin air.
We can take for granted that Macbeth is superstitious, as were most of the people watching the play in Shakespeare's time, including King James I, who was present at the first performance and for whom Shakespeare essentially wrote the play. To Macbeth and Banquo, the witches' coming and going into thin air would be perfectly acceptable and expected witch behavior.
Macbeth is mulling over in his mind what the witches said about him being king, but Banquo is a little more circumspect.

BANQUO: Were such things here as we do speak about?Or have we eaten on the insane rootThat takes the reason prisoner? [1.3.86–88]

Banquo doesn't have very long to think about it before Ross and Angus walk into the scene (oddly unchallenged, since warriors like Macbeth and Banquo have just come from battle and should be on their guard) and address Macbeth as the Thane of Cawdor.

ROSS: The King hath happily received, Macbeth . . .
And for an earnest of a greater honor,He bade me, from him, call thee Thane of Cawdor. [1.3.93, 109–110]

Macbeth is starting to believe the witches, but Banquo once again raises the question of the witches' credibility.

BANQUO: But ’tis strange;And oftentimes, to win us to our harm,The instruments of darkness tell us truths,Win us with honest trifles, to betray's . . . [1.3.132–135]

Too late. Macbeth is already thinking about his coronation, and he's talking himself into believing the witches.

MACBETH: [Aside.] Two truths are told,As happy prologues to the swelling actOf the imperial theme! . . .
This supernatural solicitingCannot be ill, cannot be good. If ill,Why hath it given me earnest of success,Commencing in a truth? I am Thane of Cawdor.If good, why do I yield to that suggestionWhose horrid image doth unfix my hairAnd make my seated heart knock at my ribs,Against the use of nature? Present fearsAre less than horrible imaginings:My thought, whose murder yet is but fantastical,Shakes so my single state of man that functionIs smother'd in surmise, and nothing isBut what is not. [1.3.138–153]

Macbeth appears to have some honor and scruples, but he's wavering in his loyalty to the king in the face of the witches' prophecies, especially since one prophecy—that he would be Thane of Cawdor—has already come true.
Macbeth shows himself to be highly susceptible to suggestion, particularly when he's already been thinking along those lines. He was also easily convinced of the witches' credibility, even though he has only one piece of evidence to support the witches' prophecies.
The witches planted a seed in Macbeth's mind, and it's starting to grow. Macbeth wants to believe what the witches told him. Macbeth's ambition is starting to override his honor and his conscience, and he's already thinking about murder, even if in the abstract.
Since Macbeth is a man of action, it's unlikely that he'll think about becoming king much longer before he actually does something about it.