Wang Lung's third son gets no love. He's almost always ignored: “There was none to trouble him, for his youngest son was a silent lad who kept out of his father's way and Wang Lung scarcely knew what he was, so silent a lad was he" (29.10).
This is the kid Wang Lung wanted to become a farmer, just like him. Wang Lung doesn't even think to ask the kid's opinion about this until four chapters before the end of the novel (30.133). And because Wang Lung doesn't notice him, we don't get to learn much about him either.
Ready? The description of the third son comes all in one big paragraph. Here goes.
And he saw a tall and slender lad, who was neither his father nor his mother, except that he had his mother's gravity and silence. But there was more beauty in him than there had been in his mother, and for beauty alone he had more of it than any of Wang Lung's children […] But across the lad's forehead and almost a mar to his beauty were his two black brows, too heavy and black for his young, pale face, and when he frowned, and he frowned easily, these black brows met, heavy and straight, across his brow. (30.144)
Got it? Basically he's a pretty but kind of gloomy kid.
Oh, yeah: this kid also really wants to go to war. After spending all day listening to the stories of soldiers, Wang Lung's youngest son starts fantasizing about the glories of warfare. And not just warfare, but a revolution. You know, the big one.
This kid isn't like his brothers. He wants more than women, respect, or money. He wants glory. He says, “I am not the ordinary young man. I have my dreams. I wish for glory. There are women everywhere" (32.54).
Now, maybe the third son's his intensity is a good thing. After all, he's the only one to escape the curse of the House of Hwang. Like his father, when his father was young, the third son is able to see the bigger picture and to hope for a different kind of future. It's ironic, then, that Wang Lung tries to stop him from becoming a soldier. It doesn't work: the third son just runs away to enlist.
As far as we can tell, he does fairly well, too. We don't find out that much about him, but we do get a few details: "Well, and he does not write a letter, but now and then one comes from the South and it is said he is a military official and great enough in a thing they call a Revolution there, but what it is I do not know—perhaps some sort of business" (34.55).
People don't even know what the revolution is because they're so blinded by their pretty desires for money, women, or comfort. All of the other people in the novel simply continue doing the same thing that people have been doing for centuries. They get rich, they get too rich, and they fall. Rinse and repeat.
What the third son is doing is totally different. This kind of revolution is something that has never happened before in China. The third son is not just part of it; he's leading it.