What is it that makes these scenes comical?
Firstly, the humour lies in Benedick looking like a fool. His reactions are crucial to the comedy. Even though Shakespeare gives him very few lines, his unspoken responses are essential to the success of the scene. The contrast between his reactions and his previous behaviour makes us laugh. Believing himself to be unseen and hidden, he is free from his stereotype as a bachelor. His ‘mask’ is off.
Secondly, the camaraderie amongst the ‘practical jokers’ is important. They are united by their secret and a sense of shared mischief and fun. The way they egg each other on, and encourage each other to even greater heights of ‘acting’ is the source of much of the scene’s humour.
Finally, the joke is also on the jokers (depending on how the scene is directed). They may be unconvincing at points; perhaps they almost fail. Will he believe them? Will he fall for it? Leonato may well be a source of amusement in this sense – see lines 135-143.