Describe Macbeth's state of mind in scene 3 in Macbeth.
Concerning Shakespeare's Macbeth, you don't identify which act you're asking about. I have a hunch you mean Act 1.3, so I'll answer accordingly.
First, whatever state of mind Macbeth is in, it echoes the witches' state of mind, since he echoes their sentiment that foul is fair and vice-versa. This is a bad thing.
Then he starts, or flinches, when the witches hail him as Thane of Cawdor and king. Banquo laughs off the witches, but Macbeth takes them very seriously and is anxious to hear more. He appears to be motivated by ambition and to have becoming king on his mind.
His flinch at such welcome news might also indicate that he is already aware of what becoming king means--he would have to kill Duncan to get the crown. Why else would he flinch? The flinch is surprising to Banquo, which suggests it is motivated by something Banquo is not aware of and does not comprehend. Macbeth may already be haunted by the thoughts that moments later horrify him--thoughts of killing Duncan (see his aside starting at line 130). Macbeth is already a mixture of fair and foul.
Macbeth is already torn, then, in Act 1.3, between what he wants and what he has to do to get it. He's obviously excited about the possibility of becoming king, but torn about what it will take to achieve the throne. He is not prepared to blindly go after what he wants, though. He is aware how horrible what he's thinking about is.
If you mean Act II, scene 3, his mind is racing as is his heart in all likelihood, since he has just murdered the king.
How Macbeth acts (and I do mean act in every sense of the word) in this scene either immediately gets him the crown or not. He could be found out if he or Lady Macbeth says or does anything to make those assembled suspicious of them.
First, the body of Duncan is discovered by Macduff, then Macbeth kills the grooms and explains it away as sudden fury at the sight of the dead king and the men accused of the deed. Suspicion must be thrown on Malcolm and Donaldbain so that they flee which makes them look guilty.
Therefore if you are refering to this scene 3, it is the fullfillment, at least for the moment of the predictionsd of Act I, scene 3.