What are three subjects Mercutio and Romeo make puns about in Scene 4, lines 44-70?
Romeo says that love "pricks like thorn"; in other words, he feels injured by love. Mercutio, however, tells him to "be rough with love / Prick love for pricking, and you beat love down." Prick means to poke, and this is the way Romeo uses the word, but Mercutio's response relies on the other meaning of prick, creating the pun. A prick is also a slang term for a penis. Therefore, Mercutio's pun implies that Romeo should have his way with love rather than allow it to hurt him.
While Romeo's friends are attempting to goad him into going to the Capulets' party, Romeo says that he is done. Mercutio responds, "Tut, dun’s the mouse, the constable’s own word. / If thou art dun, we’ll draw thee from the mire [...]." The pun is on the word dun. First, Romeo says that he is done, meaning finished, and then Mercutio uses a popular expression that essentially means "Be still," and then says that if Romeo is dun/done, then they will pull him like a stick from the mud and get him to go to the party.
Then, Romeo says that "[He] dreamt a dream tonight," and Mercutio responds that he did too. Romeo asks him what it was, and Mercutio says that "dreamers often lie" (1.4.56). Romeo replies back, "In bed asleep while they do dream things true" (1.4.57). Here, the pun is on the word lie. Mercutio means to suggest that someone might be dishonest, but Romeo changes it to the other meaning, as in to recline or lie down.