3rd Regiment, US Reserve Corps, Missouri Infantry (3 months 1861) (Union)
This regiment was organized at Turner Hall, St. Louis, Mo., May 8, 1861. It was attached to Lyons' Army of the West. Capture of Camp Jackson May 10. Provost duty at St. Louis till July 1. Three Companies sent to Southwest Missouri July 1. Expedition to Northern Missouri against Harris July 16 (7 Cos.). Action at Fulton July 17. Duty at Fulton and St. Louis till August 18. It mustered out August 18, 1861. 
Companies in this Regiment with the Counties of Origin
Men often enlisted in a company recruited in the counties where they lived though not always. After many battles,companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.
- Beginning United States Civil War Research gives steps for finding information about a Civil War soldier or sailor. It covers the major records that should be used. Additional records are described in 'Missouri in the Civil War' and 'United States Civil War, 1861 to 1865' (see below).
- National Park Service, The Civil War Soldiers and Sailors System, is searchable by soldier's name and state. It contains basic facts about soldiers on both sides of the Civil War, a list of regiments, descriptions of significant battles, sources of the information, and suggestions for where to find additional information.
- Missouri in the Civil War describes many Confederate and Union sources, specifically for Missouri, and how to find them.. These include compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- United States Civil War, 1861 to 1865 describes and explains United States and Confederate States records, rather than state records, and how to find them. These include veterans’ censuses, compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- National Park Service, The Civil War Soldiers and Sailors System (accessed 11 January 2011).