The mood at the beginning of Act 3 is playful, as Puck and the fairy banter.
By Act 2, there is a lot of trouble brewing. As a break from the high drama, we are treated to a little bit of playful banter between Puck and one of Titania’s fairy attendants. She asks Puck if he is Robin Goodfellow, and they discuss his antics playing tricks on mortals.
I jest to Oberon, and make him smile,(45)….
And sometime lurk I in a gossip's bowl
In very likeness of a roasted crab,
And, when she drinks, against her lips I bob,(50)
And on her withered dewlap pour the ale. (Act 3, Scene 1)
Although the general mood of the scene’s opening is mirthful, there is some important information given too. They discuss the fight between Titania and Oberon over her human changeling, which has implications for the forest. If the two continue fighting, the forest continues dying.
Although the craftsmen provide most of the comic relief in the play, there is a fair amount provided by Puck as well. He is playful while being pointed in his barbs. In him we learn a great deal about the fairy world, and even more about the mortals. For example, when Puck gives Bottom a donkey's head, we learn more about his character and even begin to sympathize with him and see him as more than an obnoxious blowhard.