Captain, here are the companies of Marmaduke's 18th Arkansas, including the three you asked about --

Co. A—Little Rock Grays, from Little Rock, Arkansas.
Co. B—Young Guard, from Memphis, Tennessee.
Co. C—Fletcher Rifles, from Mill Bayou, Arkansas.
Co. D—Shamrock Guards, from Vicksburg, Mississippi.
Co. E—Linden Dead-Shots, from Linden, Arkansas.
Co. F—Chalk Bluff Rebels, from Chalk Bluff, Arkansas.
Co. G—Pine Bluff Artillery, from Pine Bluff, Arkansas.
Co. H—Swamp Rangers, from Vicksburg, Mississippi.
Co. I—Burrowville Mountain Guards, from Burrowville, Arkansas.
Co. K—Rector Guards, organized at Little Rock, Arkansas.

Companies I and K were comprised largely of suspected Unionists from northern Arkansas.

The company of escaped convicts from Iowa is a new one for me. There weren't any Iowans in the Arkansas companies. I haven't researched the Tennessee company or two Mississippi companies beyond what's in the Compiled Service Records, so I don't know where all of those boys were from originally. It's worth looking into, though.

Oh, and the two Vicksburg companies retained their separate identities and organization in both the 18th Arkansas and the 3rd Confederate regiments.