Throughout the novel The Wonderful Wizard of Oz, the Scarecrow, Tin Woodman, and Cowardly Lion display the qualities they believe they lack. As the three accompany Dorothy down the yellow brick road, they encounter many obstacles. The way they handle these obstacles proves that the Scarecrow has a brain, the Tin Woodman a heart, and the Cowardly Lion courage, well before they arrive in The Emerald City.
The Scarecrow laments his lack of brains from the moment he meets Dorothy. However, it's soon clear that the Scarecrow is a philosopher and a thoughtful observer. His reasoning skills are evident in the following passage:
"This must be the Land of Oz," said Dorothy, "and we are surely getting near the Emerald City."
"Yes," answered the Scarecrow. "Everything is green here, while in the country of the Munchkins, blue was the favorite color. But the people do not seem to be as friendly as the Munchkins, and I'm afraid we shall be unable to find a place to pass the night" (Baum, 1920, Ch. 10).
Here, Scarecrow is able to reason that the change in scenery means they are no longer in the land of the Munchkins, and also that the lack of hospitality in the area might mean they won't have shelter.
The Tin Woodman's sentimental heart is also evident from the moment we meet him, yet a prior breakup scarred him so, he asked for his heart to be removed. The following passage is just one example of the Tin Woodman's tenderness, proving that he already has a big heart.
"When, at last, [the Tin Woodman] walked into Dorothy's room and thanked her for rescuing him, he was so pleased that he wept tears of joy, and Dorothy had to wipe every tear carefully from his face with her apron, so his joints would not be rusted" (Baum, 1920, Ch. 13).
Here, the Tin Woodman is overcome with joy that the tinsmiths in the woods could repair him, showing gratitude, and he feels happiness to the point of tears when he is reunited with his friend, showing how much he cares for her.
Finally, the Cowardly Lion does a fair share of whimpering and whining on the way to Oz, but he also proves his mettle in dangerous situations, such as when the Wicked Witch imprisons and starves him:
"[Every] day [the Wicked Witch] came to the gate at noon and asked, "Are you ready to be harnessed like a horse?" And the Lion would answer, "No. If you come in this yard, I will bite you" (Baum, 1920, Ch. 12).
The Cowardly Lion refuses to be harnessed by the Wicked Witch, even though she is withholding food from him in order to get him to comply. Unbeknownst to the witch, Dorothy is sneaking him food at night, preventing him from being hungry. Even so, the Cowardly Lion stands up to the witch -- even threatening to bite her -- showing courage.