In Much Ado About Nothing, what literary techniques can I compare in Act II scene 3 and Act III scene 1?
I think the main approach you can take would be to look at the role of irony and how it is played out in these two scenes. Of course, both of these scenes show the complete turn around of Beatrice and Benedick, and how easily they are persuaded that they are in love with each other. The irony here is two-fold: firstly, the massive irony comes from their mutual hatred and the way that they swear they will never become emotionally involved. Secondly, there is dramatic irony as we and the other characters know that they are presenting a fabrication of the truth to Beatrice and Benedick, who just happen to be listening to this carefully staged ruse.
Note the intense irony of Benedick's words at the beginning of Act II scene 3:
I do much wonder that one man, seeing how much another man is a fool when he dedicates his behaviours to love,k will, after he hath laughed at such shallow follies in others, become the argument of his own scorn by falling in love. And such a man is Claudio.
Benedick is so swift to bemoan the impact of love on Claudio's character, but by the very end of this scene all it takes is a few overheard comments to turn himself into that very kind of individual. Likewise, we see with Beatrice exactly the same situation. For all of her fine words and witty retorts, she is swift to fall in love and believe what she hears. Irony abounds from start to finish.