Again, no time has lapsed since the previous act. Mae, Big Mama, Gooper, and the Reverend all enter at once, looking for Big Daddy, followed by Maggie and Dr. Baugh. Big Mama says that Big Daddy has gone to bed, and remarks that he ate like a horse at dinner because he was so relieved. Gooper wants to have a family talk, and Maggie fetches Brick.
There is a fair amount of delay while Big Mama talks about how Skipper's death ruined Brick, and delays what she knows is going to be an unpleasant conversation. Finally, painfully, Big Mama is forced to understand that Big Daddy has cancer after all, and it is too far gone to be treated. Upset, she calls for Brick, her "only son" – a designation that doesn't escape Gooper's notice.
The doctor makes Big Mama take some morphine for Big Daddy for when the pain hits, and he and the reverend leave. Big Mama is still upset, and turns on Gooper and Mae, who in turn attack Brick for being a drunk. Big Mama tells Maggie that she has to make Brick pull it together so that he can take charge of the estate.
Gooper and Mae pounce on this, and pull out a briefcase of papers that his firm had drawn up for Big Daddy's property. Big Mama isn't interested in talking about who will run the place, because she insists that Big Daddy is not going anywhere. Mae and Gooper further attack Brick's competence and soundness of mind. Gooper has been overshadowed by Brick his whole life, and just wants a fair deal.
Brick finally enters, quite drunk. Now that the whole family is present, Gooper again lays out the papers. Big Mama tells them all off, on Big Daddy's behalf. She speaks tenderly to Brick, saying that it would make Big Daddy's dreams come true if he could give him a grandson "as much like his son as his son is like Big Daddy."
Mae snaps that Maggie can't oblige, but Maggie jumps to her feet and makes an announcement – she is pregnant. Mae and Gooper don't believe her for a second, but Big Mama is thrilled, and exits to tell Big Daddy the good news. After expressing their disgust, Mae and Gooper leave as well, and Maggie thanks Brick for not exposing her lie in front of the others.
Finally, after one more drink, Brick's "click" comes, and he is full of peace. Maggie says that she is ovulating, and that she will lock up Brick's liquor until he concedes to sleep with her.
Big Mama runs on, in search of the morphine – Big Daddy is now in pain. She leaves again, and Maggie continues her plan. She is going to make her lie true, no matter what. As the curtain begins to fall, she tells Brick once more that she loves him, and he says, echoing Big Daddy, "wouldn't it be funny if that was true?"
The third act finally fills the stage for the first time in the play, bringing all the characters together under the off-stage specter of Big Daddy and his illness. Big Daddy does not return, except as an off-stage shout towards the end. This bold choice of removing a key player for the entire third act was so bold, in fact, that the director Elia Kazan forced Williams to write an alternate third act for the Broadway premiere, in which Big Daddy makes a final appearance.
Williams continues the idea of Maggie the Cat in the third act, as other characters begin to pick up on this characterization – Mae, for instance, remarks that Maggie has "climbed back up in her family tree." Meanwhile, the parallel between the relationships of the older and younger Pollitt couples is further elaborated. The last line of the play, Brick's "wouldn't it be funny if that were true?," quotes Big Daddy's same remark when Big Mama likewise expressed her love. It's a little ham-fisted, this line, in how it draws the already obvious connection between the couples in big bold letters, but it makes for an effective and sucker-punching closing line.
Poor Gooper receives his first characterization in this scene, and it isn't favorable. On paper, you can easily sympathize with the older brother who is constantly over-shadowed by the golden child. Gooper is a striver and a worker, and Brick has had everything handed to him on a silver platter, even the love of their parents. But this jealousy and resentment burst through in a display of petty greed, and every character looks down on him for it – even the doctor snubs Gooper as he exits the house.
The powerless characters of the play attempt to exert some agency in the third and final act. Gooper and Mae, who have previously remained mostly off screen, now have their moment to show off their plots and schemes. But their attempt to preempt the inheritance issue is thwarted by Big Mama, who also takes her first steps towards exhibiting some agency. In Big Mama's case, however, she chooses to use her agency to support Big Daddy's opinions and wishes. She doesn't hide her partiality to Brick and her impatience with Gooper, going so far as to stand up to him. She can't stand up to Brick or Big Daddy, but she is capable of holding her own against her weaker son.
And the play ends with Maggie making use of the only option she has. Previously, she has refused to leave Brick, or take on a lover, or do anything else that would change their static relationship – she has consented merely to stay up on the hot tin roof, waiting for it to cool down. By claiming pregnancy, she doesn't quite bring the ball into her court, but she does force a play on to Brick. She can't jump off the roof or make it cooler, but she can learn how to adapt to stand it.
Nothing really happens in the entire play, plot-wise, until that last moment when Maggie claims to be pregnant. This has not been an eventful evening in the Pollitt family – just a telling snapshot of their existence at a time when underlying tensions are coming to the fore, but with nothing actually changing. Williams avoids the crutch of theatrical plot devices – and plot altogether – to create what is closer to a theater of the mind.