Summary
Proteus informs the Duke about Valentine's plan and asks the Duke not to reveal him (Proteus) as his source.
The Duke vows to intercept Valentine and prevent him from marrying Silvia. He approaches Valentine and pretends to be in love with a woman of his own, asking Valentine's advice for how to woo her. Valentine is exasperated by the Duke's questioning but plays along out of politeness, saying that he could hide a rope ladder in his coat and use it to visit his lady at her window.
When the Duke asks to try on Valentine's coat to test the theory, he finds a letter in Valentine's pocket that details his love for Silvia and his plan to run away with her. Furious, the Duke banishes Valentine from Milan.
Meanwhile, Launce informs the audience that his master, Proteus, is a knave.
Launce announces to Speed that he himself is in love, praising his lady for her ability to fetch milk and brew beer. She is toothless, drinks a lot, and has illegitimate children.
The Duke enlists Proteus's help in getting Silvia to love Thurio. Proteus feigns discomfort with betraying Valentine in this way, but the Duke assures him it is the right thing to do.
Proteus tells Thurio to assemble a band of musicians to serenade Silvia from her balcony that evening, hoping to thwart Thurio's pursuit and claim Silvia for his own.
Analysis
Act Three of the play is rife with dramatic irony, meaning that the audience is aware of information that one or more characters in the play are not.
Nowhere is this more prevalent than in the Duke's long-winded confrontation with Valentine. After being tipped off by Proteus that Valentine intends to steal Silvia away in the night, the Duke is tasked with accusing Valentine of this plan without revealing that Proteus – the only one who knows about it – was his source.
Many have criticized this moment in the play for being contrived and over the top, with the Duke essentially getting Valentine to confess his exact plan in the form of advice to the lovesick Duke (a widower). Likely, the conversation between the Duke and Valentine is meant primarily to entertain; Valentine's exasperation at the Duke's continued questioning showcases how impatient he is to carry out the very plan he has suggested for the Duke. It also, of course, underscores the dramatic irony, as the audience is aware of the Duke's plan to banish Valentine rather than simply accuse him of attempting to steal his daughter.
This act of the play also continues the developing subplot of the servants, specifically that of Speed, Launce, and Launce's dog, Crab. This, of course, is the play's central form of comic relief. In this particular act, Launce decides that he, like Proteus (his master), has fallen in love. Launce's lady, however, is markedly different from either Julia or Silvia. Launce describes what many at the time would consider a repulsive woman – a toothless alcoholic with illegitimate children.
While this description is primarily intended to simply make audiences laugh, Launce's concurrent assertion that Proteus is a "knave" is notably insightful. Launce unabashedly expresses his affection for a rather undesirable woman, while Proteus, by contrast, deceives two beautiful woman as well as his best friend, all at the same time. This contrast returns audiences to the play's unique presentation of the master-servant relationship, wherein the servants engage in silly antics but are often, also, wiser than those they serve.
