Act III of William Shakespeare's plays are always full of action which have significant consequences for the rest of the play, and Hamlet is no exception. Each of the major characters does something (or has something done to them) which significantly impacts them as well as the outcome of the play.
Polonius: in his eagerness to further ingratiate himself with the king, convinces Claudius that Hamlet's recent madness is caused by unrequited love for Ophelia. He and Claudius send Ophelia as "bait" to prove it; later, still on a mission to prove his point, Polonius is inadvertently killed by Hamlet because he is hiding in Gertrude's room to eavesdrop.
Ophelia: in this act she loses the man she loves. While she obeys her father's command to stop seeing Hamlet, Polonius also uses her to trap Hamlet into revealing the cause of his madness--a plan which does not work because Hamlet figures out the men are listening. This conversation with Hamlet is so confusing and full of contradictions that she will eventually do something quite drastic because of it.
Claudius: though he is not convinced that Hamlet's madness is due to love for Ophelia, he uses Polonius to gather more information. After listening to Hamlet's soliloquy and his conversation with Ophelia, he says that "madness in great ones must not unwatch'd go." This is the beginning of his plan to dispose of Hamlet.After he sees his foul deed played out on stage, he kneels to pray for forgiveness but cannot. The audience hears him admit to killing his brother:
Gertrude: she can no longer play the innocent after this act. She learns the truth about her brother-in-law/husband in this act, and we do not yet know whether she will remain loyal to him or to Hamlet, her son.
Hamlet: of course most of the action (and dialogue) in this act is Hamlet's, just as it is in the rest of the play. He admits to loving Ophelia--and then denies it before saying it again. Because he is always on the alert for signs of guilt in Claudius, he trusts no one. He arranges for a play to reenact King Hamlet's murder, hoping to confirm Claudius's guilt; yet even though he has had his suspicions confirmed, he still does not complete the task his father gave him. Instead he kills Polonius, thinking the man was Claudius. This is the first of many deaths to come.
Act III is full of intrigue and plotting, but it also confirms truths to the audience which had only been speculation and assumption before now.