Three Soldiers by John Dos Passos, as the title suggests, follows the journeys of three soldiers in the European theater of World War II. Despite the three soldiers' different personalities and experiences in the US armed forces, all three were dramatically changed by the war and by the military machine they're a part of.

The overarching theme, or subtext, of the book is the realities of war. The first soldier shown in the literary triptych is an ambitious young man from San Francisco who wants to become a Corporal despite his hatred for the Army. This negative sentiment toward the military is a persistent theme throughout the book. The two other main characters even go as far as deserting the Army, which is a major crime in the armed forces.

Another theme in the book is the concept of morality and immorality. Chrisfield kills his own fellow soldier due to personal animosity. It is clear that the author portrays war as a hellish phenomenon in which social constructs like morality and law are suspended. War unleashes men from the restrictions of social norms.

One of the themes that is also persistent throughout the book is the mechanisms of wartime propaganda. The Germans are portrayed as savages that need to be exterminated. Allusions to brainwashing can be seen throughout the novel, whether in the form of propaganda films or receiving punishment from fellow military officers.