"Things without all remedy should be without regard: what's done is done." How do Macbeth and Macduff apply this to 3 distinct aspects of their life?
I believe it is asking for comparisons between Macbeth and Macduff in 3 similar situations and how they apply the advice that Lady Macbeth offers in Act 3, scene 2.
Lady Macbeth says "Things without all remedy should be without regard: what's done is done" to Macbeth at the beginning of Act 3 Scene 2 when they are talking about the continual plague of thought and fear that killing King Duncan has brought to them. Lady Macbeth suggests that the actions of the past cannot be changed, so they should move on from them. And although Macbeth continues to think about King Duncan's death, he does resolve himself to move on from the past as the play continues. For example, in Act 5 Macbeth realizes that many of his men have deserted him to fight against him with the English army. Macbeth does not state any regrets over his past actions and resolves to fight without his men. He states that he "will try the last" and that he will not yield to Malcolm and Macduff.
Macduff also applies Lady Macbeth's philosophy to his situation when he learns from Rosse that his entire family and court have been murdered. His grief is brief, yet he does not stop to dwell upon the past. Knowing that he cannot raise his family from the dead, Macduff resolves to stop the tyranny of Macbeth and Malcolm agrees to help.