Diamond’s main claim in Chapter 3 of Guns, Germs, and Steel is that the Spanish were able to defeat the Inca because the Spanish had “guns, germs, and steel” while the Inca did not. This is not a very controversial claim and is really only made as a way to set up the rest of the book.
Chapter Three is meant to show us how “guns, germs, and steel” helped the Spanish to defeat the Incas. Diamond uses that phrase as a shorthand for the various advantages that the Spanish had. He says, on p. 80 in the paperback version of the book, that the
Immediate reasons for Pizarro’s success included military technology based on gunsm, steel weapons, and horses; infectious diseases endemic in Eurasia; European maritime technology; the centralized political organization of European states; and writing.
This is the major claim of Chapter 3, but it is not the main claim of the book. What is more interesting to Diamond is why the Europeans came to have the guns, germs, and steel while the Inca did not. In the rest of the book, Diamond will attempt to explain why this was so. Chapter 3, then, is meant to set up the rest of the book. It lists the immediate reasons why the Spanish defeated the Inca and encourages us to wonder about the ultimate causes that gave the Spanish their advantages.