The Duchess is in her bedroom with Antonio and Cariola.
She and Antonio tease each other and flirt as Cariola helps her prepare for bed.
Honestly, if this scene doesn't convert you to Team Antonio and Duchess, you're probably evil. Or the Cardinal. Or Ferdinand.
The three chat for a little while about marriage and sex and whatnot, when Antonio whispers to Cariola while the Duchess is distracted that they should leave the room and let the Duchess talk to herself. Antonio apparently finds this hilarious. We agree with Antonio.
The Duchess, still thinking she's talking to Antonio and Cariola, is all "oh, yes, my brothers, we really should hold off on that next child until you're BFF with them."
She sees Ferdinand, and, to her great credit, doesn't flip out: "whether I am doomed to live, or die, / I can do both like a prince" (3.2.68-69).
Ferdinand gives her his poniard again, and tells her to kill herself. Nice guy, that Ferdo.
"Whoa, hold up, you've got the wrong idea: yeah, I have a few kids now, but I'm married! I mean, probably not to the guy you would have picked for me, but, still, married! Brazen Hussy I am not!"
This doesn't calm Ferdinand down in the way the Duchess is hoping. He launches into a diatribe about how she'd better hide this guy really carefully, because when Ferdinand finds this husband of hers, he's going to kill him.
The Duchess calls him out on how unreasonable this all seems: "Dude, people get remarried. Women get remarried. I got remarried. It's not just legal, it's totally normal, so what's the problem?"
After furiously telling her that she has utterly destroyed her reputation, Ferdinand promises to never see her again and storms out.
Antonio and Cariola, having heard what's gone down from their hiding spot, come back in.
They have just enough time to communally decide that they're in real trouble before Bosola knocks, and Antonio once again hides.
Bosola tells the Duchess that Ferdinand has just taken off for Rome, and wants to know what's up.
The Duchess, again, is remarkably quick on the draw: Antonio, she says, has been mishandling her money, and as a result has really messed up her and Ferdinand's finances. A wee bit of misdirection, you see.
After Bosola has left and Antonio once again comes out of hiding, the Duchess tells Antonio that he's got to high-tail it over to Ancona and lay low for a while.
In the meanwhile, she explains, she's going to have to make it look like she's firing him for mishandling her accounts.
Bosola and the other officers of the court come in, and the Duchess and Antonio play out a scene where she very dramatically accuses Antonio and then fires him, after which he leaves.
The Duchess turns to her officers, and asks them what they think of Antonio.
The officers, figuring that Antonio is not a guy to be siding with right now, all reply, "oh, yeah, that Antonio, I always knew he was a bad dude, coulda seen that coming a mile away." They leave, and she poses the question to Bosola.
Those guys are unprincipled jerks, Bosola says: "Antonio was a good guy, a great steward, he always had your back, and that you just threw him under the bus means that you're just as bad as every other corrupt politician."
The Duchess, although she draws out the charade for a little while, is thrilled to hear this. She figures that somebody who stands up for Antonio is worth trusting, and tells Bosola that Antonio is her husband.
Bosola acts shocked and impressed that "some preferment in the world can yet / Arise from merit" (3.2.281-81).
The Duchess swears him to secrecy, and asks him to take charge of all her money and jewels and to bring them to Antonio in Ancona. Uh oh.
Bosola advises her to travel to Loretto, pretending that she's making a religious pilgrimage.
The Duchess agrees, saying to Bosola, "your direction / shall lead me by the hand" (3.2.308-9). This is the point where the audience goes from Kind of Worried to Duchess, You In Danger, Girl.
Cariola isn't on board with Bosola's plan, but the Duchess tells her she's being stupid and to go prepare for their departure. They leave.
Bosola, now possessing the information he's been after for years and having finally won the Duchess's trust, announces that he must immediately update Ferdinand, and mentions his dissatisfaction with his job as a spy.