Though there is much debate about Gertrude's complicity in the death of her first husband, it seems to me Gertrude's allegiance is another thing that shifts in Act III. As mentioned above, Hamlet does reveal to her that he is only "mad in craft" and asks her to keep it to herself. Her loyalty is put to the test as soon as she leaves her chamber and sees Claudius, her current husband. When asked, she tells him Hamlet is, indeed, crazy and has even gone so far as to kill Polonius. That seems to be the moment when she chooses loyalty to Hamlet over loyalty to Claudius.
The duplicity of the first scene in which Claudius, Gertrude, Polonius, Ophelia, Rosencrantz and Guildenstern all conspire against Hamlet for the king's evil intent is reversed in the second scene as Hamlet conspires against them, but now with good intentions rather than evil. Truly, it is a Dumb Show ironically that the court watches as Hamlet has his friend Horatio scrutinize the reactions of the others for indications of their guilt in the murder of King Hamlet.
Another shift in Act 3 is Hamlet's eventual revelation to his mother that his not truly mad but has only been acting mad. He asks his mother to keep this confidence, suggesting that he trusts her. This conversation takes place at the end of the "closet scene" where he has chastised her for her beahvior with Claudius and she admits that he has made her see the truth of her actions. Perhaps she only says this to get Hamlet to stop yelling at her, but Hamlet probably takes her at her word, thus inspiring a new level of trust and honesty between them,.
The topic moves from this acknowlegdement of the ghost of Hamlet's father and seeking revenge for the father's inappropriate death to the love Hamlet has for Ophelia. In fact, just about every possible interested party weighs in on the issue. The fathers of both plot to try to overhear a conversation between Hamlet and Ophelia. Ophelia herself participates by purposing to engage Hamlet in conversation. This is one possible shift to identify.
A second potential shift occurs as Hamlet sets and stages a play that identifies the circumstance he thinks happened to position his father to be killed. This informal and fun portrayal of circumstances is much different than just coming out and saying what he feels or dreads about the situation.