In chapter three of To Kill A Mockingbird, Scout fights with Walter Cunningham. Jem makes a humorous comment after he breaks up the fight: 

Catching Walter Cunningham in the schoolyard gave me some pleasure, but when I was rubbing his nose in the dirt Jem came by and told me to stop. "You're bigger'n he is," he said.

Although Jem is probably being serious, his comment comes across as humorous. No doubt, Scout is bigger than Walter, but the fact that Scout is a girl and beating up a boy is a humorous event. Scout fights like a boy. She is a tom boy. Jem has always treated Scout like a boy. His comment is humorous because he never mentions that Scout is a girl fighting a boy. He only comments that Scout is bigger than Walter. This creates a smile for the reader. 

When Scout mentions that Walter Cunningham is nearly as old as Jem, her comment is humorous:

"He's as old as you, nearly," I said. "He made me start off on the wrong foot."

She is a tough little girl who does not take any sass form anyone. The reader can only smile at the actions of Scout.  

Another humorous event from chapter three happens when Scout gets in trouble for ridiculing Walter Cunningham for pouring syrup  all over his food. Calpurnia orders Scout to report to the kitchen. Calpurnia disciplines Scout for her rude behavior toward her guest, instructing her on how wrong it is to judge company:

"There's some folks who don't eat like us," she whispered fiercely, "but you ain't called on to contradict 'em at the table when they don't. That boy's yo' comp'ny and if he wants to eat up the table cloth you let him, you hear?"

Scout becomes angry at Calpurnia for her strict discipline. Scout threatens Calpurnia. The reader finds Scout's comments humorous as she is feeling sorry for herself:

I told Calpurnia to just wait, I'd fix her: one of these days when she wasn't looking I'd go off and drown myself in Barker's Eddy and then she'd be sorry.

Here, the reader smiles at the comments of Scout. She is cute when she is pouting. Of course, she is not going to drown herself. She is being overly dramatic. Her actions cause the reader to find humor in the surrounding events. 

Another humorous event from chapter three happens when Scout asks her father if she can leave school after her first day. She is quite serious. She mentions that she can be like Burris Ewell who only reports to school on the first day and then never returns. Although Burris's situation is a serious one, it is humorous to think that Scout is thinking of quitting school after only the first day. Truly, Scout has had a difficult day at her first day of school. Again, Scout causes the reader to smile at her dilemma.