It is important to note that Shakespeare uses different ways of speaking to develop his characters; nobility speak in iambic pentameter and commoners tend to speak in prose..this includes nobles who are doing or thinking something very not noble.
Macbeth's arrival in the scene is hearalded by his comment to Banquo:
"So foul and fair a day I have not seen" (l. 39).
This line serves more purpose than just idle chatter with Banquo (who is undoubtedly Macbeth's greatest victim of betrayal). First, the line communicates an overriding theme of many of Shakespeare's plays: things are not what they seem. Macbeth is not the hero he has, so far, proven himself to be. The witches might be homeless women, or they might be capable of "vanishing into thin air." Banquo is not as dearly loved by Macbeth (we find out later) as the audience believes him to be. Very little in this play is as it appears in the beginning.
Additionally, Macbeth's introductory line in the scene alludes to the paradox within his character: he is a war hero, but he is not of heroic character; he becomes Thane of Cawdor, which puts him in charge of vast territories (it was a most coveted position among the nobles), but Macbeth proves himself capable only of following his wife. The person "wearing the pants" in their marriage becomes clear in scene 5.
Herein lies the conundrum of the scene, and the brilliance of Shakespeare's writing. The reader has become uncomfortable. Is Macbeth a good guy, or a bad guy? Will he uphold the tenets of heroism, or are the witches manipulating fate.....or is Macbeth going to destroy himself regardless of what fate has in store?
Here is another example of how Macbeth's lines in scene 3 cause ambivalence, or tension, for the audience. His lines bounce back and forth between the noble iambic pentameter and the lowly use of prose. Macbeth hasn't done anything ignoble yet, but his bouncing back and forth between speaking in poetry and prose makes it clear that Macbeth is, at the very least, capable of behaving in a less than noble manner. His lines in scene 3 provide foreshadowing; his character becomes more and more clear as the play progresses. Macbeth's greatest flaw, I would argue, is lack of moral backbone. Although he admits to "vaulting ambition" as his flaw, it is clear that he does not know himself. He is a very poor judge of character, and that is what "haunts" him throughout the play. His wife pushes him around through her own ambition. He has the friend he loves most murdered. He goes against God by defying the law of primogeniture - because his wife told him to do it. Macbeth is easily flustered, easily distracted, and makes increasingly poor decisions. He is alternately frightened and possessed by megalomania. In essence, he becomes everything that an effective king is not.
In short, Macbeth's lines underscore that his character is at the tipping point between good and evil, and his lines serve to clarify the most important themes of the play.