Symbolism, Imagery, Allegory
Like Dorothy, the Scarecrow, the Tin Woodman, and the Lion embody traditional American values. The Scarecrow symbolizes intelligence and cunning; he's always coming up with good ideas that help the group out of bad situations. The Tin Woodman symbolizes compassion. He's the conscience of the group, always making sure that their actions don't hurt anyone else. The Lion symbolizes bravery. In a pinch, he's always willing to turn around and fight to the death.
What's interesting about these three characters as symbols is that they are plagued by self-doubt. They see themselves as incompetent and incomplete, which causes them a lot of personal pain. The sense of inadequacy also makes them more relatable as characters since many readers will have struggled with similar feelings. We've all had moments where we wished we were just a little bit smarter, or more compassionate, or more courageous, right? It's a normal human failing. And often others see in us those qualities we think we lack.
Blind to the traits and skills they already possess, the Scarecrow, the Tin Woodman, and the Lion expect to receive a brain, a heart, and courage as rewards for a job well done. The Wizard, hack that he is, presents them each with prizes: a brain consisting of "a measure of bran…mixed with a great many pins and needles" (16.9); a "pretty heart, made entirely of silk and stuffed with sawdust" (16.21); and a dose of courage that appears to be more or less a shot of whiskey. All three are empty objects symbolizing traits that the characters already have.
So what's the takeaway? First of all, the characters' focus on the Wizard's material gifts is misguided. If they would only look inward, they would recognize their strengths. Instead they seek two unnecessary things: approval from an outside source and material rewards that they don't need. The author's message is that true happiness and achievement is not about the things you acquire. It's what's on the inside that really counts.