In Macbeth, state three important events that happened off stage which serve to advance the plot.

Certainly, the murder of Duncan is a major event that moves the plot forward, and it is does not occur onstage.  During Act 2, Scene 2, Lady Macbeth says, "He is about it," letting us know that, at this moment, Macbeth is in Duncan's bedchamber killing the king.  Then, when he arrives in her room, he says, "I have done the deed."  Obviously, it is this event that inaugurates Macbeth's tragedy by being the first...

Certainly, the murder of Duncan is a major event that moves the plot forward, and it is does not occur onstage.  During Act 2, Scene 2, Lady Macbeth says, "He is about it," letting us know that, at this moment, Macbeth is in Duncan's bedchamber killing the king.  Then, when he arrives in her room, he says, "I have done the deed."  Obviously, it is this event that inaugurates Macbeth's tragedy by being the first step in his descent into brutality and tyranny.

Next, Macbeth's murder of Duncan's chamberlains, the guards whom he and Lady Macbeth framed for Duncan's murder, occurs offstage and advances the plot as well.  In Act 2, Scene 3, Macduff discovers the king's body, and Macbeth and Lennox exit the scene to go and see it.  After they return, Macbeth says, "O, yet I do repent me of my fury, / That I did kill them" (2.3.124-125).  We know, at this point, that he killed the guards when he was in Duncan's room.  He likely killed them in order to make sure they could not proclaim their innocence or say what they might have seen in the night.  Now, there is no one to question, no one to blame, and everyone in the house becomes suspect.  This event initiates, rightly, Macduff's and Banquo's suspicion of Macbeth.

Finally, Lady Macbeth's suicide in Act 5, Scene 5, takes place offstage and helps to advance the plot in that it forces Macbeth to come to terms with the meaninglessness of life, of his life in particular.  He and Seyton hear a scream, and when Seyton returns after investigating it, he tells Macbeth, "The Queen, my lord, is dead" (5.5.19).  This leads into Macbeth's most famous soliloquy of the play where he laments the pace and pettiness of life, its intense emotion underwritten by its lack of meaning or purpose.  He enters his battle with Macduff with these thoughts.