Most units stayed together through the war. A few units early in the war and many toward the end were consolidated with other units and when that happened Company designations could change. Here is a fairly detailed history of this unit:

http://mississippiscv.org/MS_Units/3rd_MS_CAV.htm

I don't see any reference to a Col. Ferris. Looks like the regiment was commanded by Col. McGuirk thoughout most of the war.