Set in the daytime, Act III opens in the drawing room with Voynitsky and Sonya seated as Yelena paces in thought. The professor has called a meeting at one o'clock. After grumbling about Serebryakov, Voynitsky attacks Yelena, describing her as ready to "drop from sheer laziness". Yelena replies that she is dying of boredom and is without an idea of what to do. Sonya produces a number of possibilities: help with the estate, teaching, nursing, and so on. Yelena cannot imagine being interested; only people in ideological novels undertake such work.

For Sonya, Yelena's idleness is "infectious"—her effect on Vanya, herself, and the doctor, all of whom having deserted their work to follow her, make this clear. Yelena is certainly a witch. Voynitsky adds that she must have the blood of a mermaid and should flee the estate by diving into a whirlpool with some water sprite. Yelena is enraged: Voynitsky offers to pick her a bouquet of roses in apology and exits.

Sadly Sonya then puts her head on Yelena's breast and once again begins pining for the doctor. Having loved him for six years, she has lost all pride and confessed her love to everyone except Astrov. Remarking that he is a strange person, Yelena resolves to find out what he thinks of Sonya, using his cartograms (or ecological maps) as a pretext. Sonya pauses for a moment, wondering if uncertainty is better and exits to fetch him.

Yelena then delivers a soliloquy, ruminating on how Sonya cannot help falling for such a "fascinating" man amidst such boredom and admits her own fascination for the doctor. Recalling Vanya's quip about the mermaid, she wonders if she should flee with him—her conscience, however, prevents it. Indeed, she already feels guilty, ready to prostrate herself before Sonya and weep.

Astrov then enters with a cartogram and proceeds to explain the progressive degeneration of the region. Forests, settlements, and wildlife have disappeared in the "downhill struggle for existence"; instead of progress, the same swamps, diseases, and disasters remain. It is clear to Astrov that Yelena is uninterested. He breaks off, and she cross-examines him with regards to Sonya. Astrov does not love her.

He is, however, convinced that the subtext (or underlying subject or theme) of Yelena's cross-examination is her own desire and that she is finally responding to his longtime advances. Much to Yelena's dismay, he embraces her passionately and insists upon arranging a rendezvous. Yelena resists, and suddenly Voynitsky enters unseen. For a moment, Yelena will then relent, laying her head on Astrov's chest. Just as the doctor proposes a rendezvous, however, she sees Voynitsky and disengages herself from his arms. Astrov chats about the weather, noting that the days are getting shorter, and he exits. Nervously, Yelena insists that Voynitsky prevail upon her husband for an immediate departure from the estate; a shaken Voynitsky replies that he has seen everything.