Rob, according to a transcription I've looked at, of the CSRs of the 3rd Confederate Infantry, the 3rd and 5th Confederate Regiments were consolidated from February 1863 to April 1864. There is a record of staff appointments to the consolidated field and staff made on February 1, 1863 -- presumably the date of formal consolidation. The consolidated field and staff was disbanded and the staff officers reassigned to their respective regiments on April 1, 1864, so I presume that's when the "un-consolidation" became effective.

I regret that I've never found the time to personally research Marmaduke's 18th Arkansas after January 1862, when it became the 3rd Confederate. Maybe one of these days.....