MACV divided South Vietnam into four Corps Tactical Zones for better command and control of the combat forces assigned to MACV. MACV had three corps-level headquarters commanding the forces in I CTZ (III Marine Amphibious Force); II CTZ (I Field Force); and III CTZ (II Field Force). IV CTZ did not have a corps-level headquarters and was directed by MACV directly, hence your grandfather’s dual roles within IV CTZ and MACV J-3.
Your grandfather’s duties would have encompassed the planning and logistics support for any combat forces assigned to IV CTZ. Such forces would have included US Army, US Navy, Army of the Republic of Vietnam (ARVN), and Allied forces. An example of the kinds of operations performed in IV CTZ were the numerous ops conducted jointly by elements of the US 9th Infantry Division and the Navy’s Riverine Force in the Mekong River Delta. An operations officer would have had to coordinate activities among these disparate units, to monitor the progress of these operations, provide addition combat and support elements as needed during the operations, and ensure logistics support to these often widely separated commands. Your grandfather would have been a very busy soldier indeed.
I hope you find this information helpful.
Operations would be J3 - This group would direct what everybody else did. Training would be J2 - This group trained everybody on how to do their jobs. Supply would be J4 - This group made sure everything needed to do the job was made available Beans, Bullets and Band-Aids. The Military Assistance Command - Vietnam (MACV) pointed the South Vietnamese military in the right direction within Corps Tactical Zone 4 (CTZ IV) which was the Mekong Delta.
Looks like your grandfather supervised just about every job except J1 and J5 keeping records and helping the civilian population.