In the futuristic society that Ender lives in, couples are only really allowed to have two children, and to have a third is against the rules. If you are a "third" child, then you are looked down upon by society; you are a bit of an outcast, and are ostracized, teased, discriminated against, and treated like a strange fluke. It's almost like a swear word, calling someone a third, it is that much of an insult. And, when Stilson calls Ender a third in chapter one, he is insulting him. But, it is like a call to action for Ender. It rouses his fighting instincts, and helps motivate him to fight. He realizes that he is a third, and that he is going to have to prove that being a third is not a position of weakness, but rather a position of strength. So, he fights. He fights and wins, and the people watching Ender see him and realize that he is ready for training. Ender fighting Stilson over being called a third is the final test that he passes that launches him into battle school, and in the end, into history books. It is the defining event that sets Ender on his journey to greatness. So, Ender took a bad thing, and worked it to his advantage, a trait he shows quite often in the book.