My initial impression of Macbeth is only strengthened by his behavior in Acts 3, 4, and 5. Once he is overcome by ambition, a moral weakness, his behavior becomes completely despicable. It becomes even more despicable in the remainder of the play as he sends various assassins and soldiers to murder Banquo, his son, and every member of Macduff's household. Macbeth degenerates so completely, he murders children without hesitation. He may have been a brave and loyal soldier in the very beginning of the drama, but Macbeth's horrid deeds negate that.
Lady Macbeth's behavior in Acts 3, 4, and 5 does add some complexity to my first impression of her. In Acts 1 and 2, she gives the impression of being hard, cold, scheming, manipulative, well organized, and even more ambitious than her husband. She seems incapable of tenderness or pity, except for the subtle and intriguing remark about Duncan's looking like her father as the King slept moments before his murder. Once Duncan is murdered, however, she continues to exercise control of her husband and manage the events following the discovery of Duncan's body, at least for a while.
My impression of Lady Macbeth does not change in Act 3, but her subsequent emotional disintegration in the remainder of the play does change my assessment of her. I feel no sympathy for her suffering and guilt, but she clearly is not as strong or unfeeling as she first seemed to be.