The themes of this act include Appearance vs. Reality, honor, duty and responsibility and twists of fate.
One of the main themes in this act is Appearance vs. Reality. There's are many examples to this. For starters, the part where the priest doesn't know how to take Ophelia's death because it looks like it could be either an accident, or a suicide, he doesn't really know and neither does anyone else.
Another example mod this is the dueling scene between Hamlet and Laertes. It looks like it is just a fun playful fight, but in reality, this fight is actually very dangerous. It leads into another theme of honor. Laertes is trying to restore the honor that he thinks has been lost due to what Hamlet has done and he feels like it is his duty, or responsibility to bring this honor back.
Twists of fate also play apart in this act as well. Basically all the main characters die, it had been there fate all along, no one just knew it yet.