Think this might be your Marmaduke:
Luther A. Marmaduke who enlisted on 10/24/1861 at Richmond County, VA as a Private.
On 10/24/1861 he mustered into "K" Co. VA 9th Cavalry
He was Drowned on 12/21/1863 and is buried in Cedar Hill Cemetery, Covington, Va.
(Drowned while fording Jackson River.)
Promoted Sergeant 11/15/1863
No luck with the other two.
Regards - Anne