What is happening to Macbeth's sense of mortality in Act 3 of Macbeth?
Macbeth examines his own mortality much more in Act 5 of Shakespeare's Macbeth than he does in Act 3. That's when his wife dies and when he discovers the witches have been manipulating him. He knows he's going to die at that point.
In Act 3, I suppose, he indirectly speaks of mortality when he exclaims, after the appearance of Banquo's ghost, that there was a time when you could kill a man and the man would stay dead. I suppose mortality doesn't seem so permanent to Macbeth at this point.
We don't see much of Macbeth's view of mortality here, however, because he deflects the trouble he's in and changes his focus to Macduff. He employs what today we would call defense mechanisms. Lady Macbeth seems to realize his fit in the presence of the thanes signals their failure, but Macbeth just directs his attention to Macduff. He is persistent and he is a survivor.
He does seem to be more at ease with killing others, however, in Act 3. He orders the deaths of Banquo and his child, Fleance, with much more ease than he kills Duncan in Act 2. The value of life seems to have lessoned for Macbeth.