I'm wondering if those three troopers died of wounds received earlier fighting Marmaduke's raid.
I sympathize with your being a distance away from the "Supplement to the 'O.R.'" Since Broadfoot is a publishing firm in Wilmington, NC I seriously doubt this 100-plus set of books is available online. You may wish to contact them and see if they will sell you just the volumes containing the Union and Confederate units--volumes 34, 35, 36, 37, 38. Whatever you find out, would you please let me know the outcome?