In the book Our Town, what did Emily Webb learn in act III?
By the time Emily learns what she learns about life, as seen in Act III, she is dead. And that's part of the point of what she learns: there is no way to know about the life we live as we are living it.
This part of the play, when Emily sees how much of her life she had missed while she is alive, is so sensitively and emotionally written that any paraphrase here would not serve you well. This will sum it up best for you:
...Live people don't understand, do they?
No, dear not very much.
They're sort of shut up in little boxes, aren't they?
Here's a little Zen story that illustrates the same point: A Zen student who had meditated for years in order to reach the state of sartori, of being always present and aware, was sure he had finely attained true enlightenment. He was so excited about it that he made an appointment with the Zen master and went to visit him in his humble home.
"So," said the master, calmly, "I understand that you have reached enlightenment."
"Yes!" exclaimed the student, "Yes I have!"
"Tell me please," said the master, "when you took off your wooden shoes before entering my home, where on the steps did you place them? On the left or the right?"
The student was unable to answer the question, and the master sent him on his way.