After reading Act 3, why aren't Macbeth and Lady Macbeth happy being king and queen?
In adding a few details to the previous answer, I would like to begin with the idea that very few people are satisfied with what they have. Macbeth is no different.
First of all, power makes him forget a few key prophecies the witches tossed to Banquo, mainly that Banquo's sons will be king. Only after he kills Duncan does he realize that "to be thus, is nothing; but to be safely thus" (III, i, 51-52).
In order to be safe, he has to continue killing, beginning with his best friend and his son. Unfortunately, Fleance escapes, heightening Macbeth's fear that "For Banquo's issue have I filed my mind; / For them the gracious Duncan have I murder'd; . . ." (III, i, 68-69).
With this realization, Macbeth not only becomes unhappy and angry, but he begins his descent into insanity and evil. His embarrassment at his own coronation banquet can never be undone. Other than covering for his insane behavior, Lady Macbeth has no more impact upon Macbeth's activities. Their apparent bond has been broken.
The final scene of this act confirms Macbeth's undoing. Instead of a kingdom full of admiring and loyal thanes, a real rift has developed among the men. This divide is recounted by Lennox and another nobleman. They note that Macduff has denied Macbeth's invitations to the castle on two occasions, thereby placing him in direct conflict with Macbeth. He has fled to England to reunite with Malcolm and hopefully find that "...a swift blessing / May soon return to this our suffering country / Under a hand accursed!" (III, vi, 8-10).
It is clear that Macbeth and his wife are not enjoying their power. She is watching her husband descend into insanity and secrecy while he loses both his friends and the support of the nation.
Macbeth has begun to realize that his time as king is limited. He knows that the witches proclaimed a long line of succession to Banquo, and his failure to dispose of Fleance has him worried to the point of hallucination. Banquo's ghost makes a couple of appearances at Macbeth's banquet, and Macbeth speaks aloud to it in front of all of his guests. Lady Macbeth is left to try and come up with an explanation for Macbeth's odd actions.
In addition, the absence of Macduff at the banquet has Macbeth assured that Macduff knows something is up. Not only does Macbeth now have to be concerned about Fleance eventually starting a line of kings, but Macbeth's own noblemen are out building armies to oppose him. The only solution he can think of is to revisit the witches for a clarification to his original prophecies.