There were four formations in the Union Army designated as III Corps (or Third Army Corps) during the American Civil War. Three were short-lived: * In the Army of Virginia, a temporary designation of the command better known as I Corps (Army of the Potomac):: * Irvin McDowell (June 26 – September 5, 1862); * James B. Ricketts (September 5–6, 1862); * Joseph Hooker (September 6–12, 1862) * In the Army of the Ohio: * Charles C. Gilbert (September 29 – October 24, 1862) * In the Army of the Cumberland: * Charles C. Gilbert (October 24 – November 5, 1862)