Act III of Much Ado about Nothing highlights the motifs of communication and counterfeit. It is in this act that the two parallel storylines - Beatrice and Benedick/Hero and Claudio - are pushed forward into their conflict and climax. These actions take place because of the two motifs, and particularly the interaction of the motifs. It is through the use of counterfeit communication that Beatrice, Benedick and Claudio are deceived. It is the poor communication of Dogberry that prevents Claudio from quickly discovering that he has been misled. It is the counterfeit of Margaret that also prevents the mishap from being cleared up.
Shakespeare here is illustrating the nature of social pressure. He is showing that truth is subjective, easily influenced by perception, and that people in general put little faith in it. People are more concerned with looking good than being good, and are eager to believe what they suspect, rather than what is true. In this way, people are not only led astray by deceit, but deceitful as well -- whether for positive (Beatrice and Benedick) or negative (Hero and Claudio) reasons.