Act III Scene I opens in Lady Wishfort's home. Lady Wishfort is in her dressing room with her maid Peg waiting for Foible to help her get dressed and put on makeup. Lady Wishfort is haughty and impatient, ordering Peg around and confusing her. Ms. Marwood enters the dressing room, having beaten Foible to the house as the latter had feared. She indeed tells Lady Wishfort that she saw Foible with Mirabell in the park. This angers Lady Wishfort, and when they hear Foible approaching the room she sends Ms. Marwood to hide in a closet, reading books if she likes, while Lady Wishfort chastises her woman.
Foible enters the room, having been summoned by Peg, and immediately begins to forward Mirabell's scheme, recounting to Lady Wishfort how she showed Sir Rowland a picture of Lady Wishfort and he "worshipped" it. When Lady Wishfort asks about Foible having spoken to Mirabell in the park, Foible covers quickly, saying he had stopped her and taunted her about her pay and about trying to set up Ms. Millamant with his uncle, Sir Wilful Witwoud. Lady Wishfort plays along in perfect unawareness, calling Mirabell names and thanking Foible for what she had done. The conversation then turns back to Lady Wishfort's makeup, which has cracked "like an old peeled wall" during Lady Wishfort's energetic outbursts about Mirabell. Foible and Lady Wishfort discuss the optimal way to interact with Sir Rowland, deciding that it is best to play a little hard to get ("a little scorn is alluring"). Lady Wishfort exits to get dressed in a different room upstairs, and Mrs. Fainall enters the room just on her heels.
With Ms. Marwood still quite in earshot in the closet, Mrs. Fainall begins to tell Foible that she knows some of Mirabell's scheme, spelling out rather clearly that Mirabell's servant will be impersonating Sir Rowland, and that she was worried Foible would get in trouble after Ms. Marwood saw them in the park. Foible assures Mrs. Fainall that all is well and thanks her for her care. She asks Mrs. Fainall to tell Mirabell of her success, saying that she doesn't want to get caught again talking to him by Ms. Marwood, and even throwing in that she thinks Ms. Marwood desires him but that he does not feel the same way about her. Mrs. Fainall departs to tell Mirabell and Foible hurries upstairs to help Lady Wishfort dress.
Act III Scene II now focuses on Ms. Marwood, alone in Lady Wishfort's closet. She gives a resentful monologue, having heard all Foible said about the Sir Rowland plot and about his feelings for her. She is pleased with having secret knowledge on so many characters, however, calling herself a "master-key to everybody's strong-box." Lady Wishfort now re-enters, apologizing for having left her friend, reasoning that she is so flustered because of the situation with Sir Rowland and because Sir Wilful Witwoud will arrive any moment. Ms. Marwood encourages Lady Wishfort to think more about marrying Sir Wilful to Ms. Millamant, and Lady Wishfort says that she will. Foible now enters the room, announcing that Anthony Witwoud and Petulant have arrived to dine with Lady Wishfort, and Lady Wishfort reminds her lady that they also must prepare for Sir Wilful to arrive before lunch and therefore join them as well.
Act III Scene III takes place soon after, elsewhere in Lady Wishfort's house, and opens on Ms. Marwood, Ms. Millamant, and the servant Mincing. Ms. Millamant talks heatedly about an unpleasant conversation she has just had with Witwoud and Petulant, and Ms. Marwood bitingly tells her she should just cast away her other suitors and "own Mirabell" in public, since everyone is able to tell what has been going on. They bicker, Ms. Millamant denying the relationship and laughing falsely. Ms. Millamant calls for a song that she likes to be sung by a lady in the next room.
As the song ends, Petulant and Witwoud enter the room with the ladies. Since Ms. Millamant had left their prior conversation due to their arguing, the four young people now talk about argument itself and about the relationship between learnedness and romance. Sir Wilfull now enters, having obviously just arrived at the house from far off, with a footman in tow. Sir Wilful inquires about Lady Wishfort and, upon finding her to still be dressing, sends the footman to announce his arrival. Sir Wilfull now attempts to introduce himself to Ms. Millamant, Ms. Marwood, Witwoud, and Petulant, not recognizing that Anthony Witwoud is his half-brother. When this fact is discovered, Sir Wilfull scolds him for not greeting him in a more friendly manner as a brother, and when Witwoud downplays this by saying it isn't the fashion here, Sir Wilfull rants about him being a fop. Ms. Marwood inquires about his plans to travel, and Sir Wilfull says that he doesn't know where he will travel but he knows that he must because when he decides something he always follows through.
Lady Wishfort and Fainall now enter. Lady Wishfort welcomes Sir Wilfull warmly, upon which he snarkily comments that he was afraid she, like Anthony, would give him a cold welcome as a relation. Ms. Marwood and Fainall split off to converse apart from the group, and they are left onstage when Mincing summons everyone to eat. Ms. Marwood has evidently uncovered the plot she has recently discovered to Fainall, including the implication that Mrs. Fainall had also once been involved with Mirabell. Working off of Fainall's vengeful anger, Ms. Marwood outlines a plan for Fainall in which he threatens Lady Wishfort that he will leave his wife Mrs. Fainall. Because Mrs. Fainall is Lady Wishfort's daughter and Lady Wishfort could not bear that kind of social impropriety on both of their names, Mr. Fainall will then have the leverage to ask for Millamant's 6,000 pound inheritance. Fainall plans to get Sir Wilfull drunk so that he will not get in the way, and Ms. Marwood will write a letter exposing the plot to be delivered to Lady Wishfort right when she is meeting with the imposter Sir Rowland. The scene ends with Fainall pledging, if nothing else, that he will at least be able to ruin his wife and take her money, and Ms. Marwood remarks that he should now believe that she hates Mirabell.
In Act III Scene I, we see Foible again carrying out Mirabell's scheme in a seemingly very comfortable, even playful, manner. Perhaps this is because Foible feels finally able to wield and exert power over her upper class employers, even while being employed by another upper class person in the scheme itself. Like Waitwell remarks at the end of Act II, Foible's life has changed greatly in the past day, having been employed in an exciting plot, married, and having the privilege of conversing secretly with other upper class people like Mrs. Fainall who are kind to her because of her position of power in the plot.
Lady Wishfort is introduced in this scene, and is one of the most outlandish and comical characters in the play. She has apparently been dressing, putting on makeup, and drinking for the first two acts of the play, and continues to do so slowly, demanding help from servants at every step of the way, until the last scene of Act III. Again, the theme of aging is very important to Lady Wishfort, and she self-deprecatingly remarks on her cracking makeup and the need to paint oneself to be able to match the painted portrait that her supposed suitor, Sir Rowland, has seen. In a bit of sad dramatic irony, she seems genuinely pleased with Sir Rowland's courtship, pursuing it perhaps for status, but not for money as the other romances in the play all seem to involve.
Perhaps in the most important scene of dramatic irony in the play, if not in Restoration Comedies as a genre, Ms. Marwood's hearing Foible's retelling of Mirabell's scheme from within Lady Wishfort's closet is a brilliantly planned and executed plot element. The fact that Ms. Marwood overhears the scheme and is not told allows her to react authentically and devise her own plot in retaliation which will lead to the play's climax. Furthermore, the fact that Foible goes into both Mrs. Fainall and Mirabell's prior relationship and Mirabell's lack of desire for Ms. Marwood adds comedy to the dramatic irony and gives Ms. Marwood additional anger and fodder for her plan.
The song, called to be played by Ms. Millamant early in Scene III, is comedic in that only the two feuding women can be seen onstage, and the song centers around not wanting a lover for one's own happiness but but for the feeling of having something others want. This private interlude comically underlines the prior point made about the not-so-pious relationships between women, especially when only the two are together and social propriety starts to break down. It is up to directorial decision what the women do while the song plays, but it is clear that they should be the only two onstage together, and remain without talking or dancing while the song plays itself out.
Act III solidifies to the reader and audience that there are really no protagonists in the play. Though Ms. Marwood has been slighted by the man she desires, her retaliation is to devise her own plot, along with the married man she is mistress to, to extort money from the same rich Lady that Mirabell attempts to. Lady Wishfort, though more honorable in her romantic aims, wields Ms. Millamant's inheritance over her and disparages her servants to the point where even Foible turns to ugly scheming. Because all of the characters not involved in these money-focused schemes are so foolishly fashion-focused, the audience is not "rooting" for any particular character to succeed, leaving the climax open for a moralistic resolution either in the success or failure of Mirabell's plan.