The only Robert S. Crum I found serving from a Missouri formation was the one who served in the Maries County Battalion of the EMM, enrolling in Oct. of 1863 in Vienna. Robert S. Crum later enrolled in feb. 1865 in the Osage & Maries co. VMM (Veteran Missouri Militia).
There wasnt a Robert Crum, or Crumb, in the 3rd Missouri cavalry, or any other Missouri unit. I hope this helps.