Complex sentence: The ROK Army was formerly organized into 3 armies: the First Army (FROKA), Second Army (2nd OC), and Third Army (TROKA), each with its own headquarters, corps, and divisions.
Split into simpler sentences: The ROK Army was formerly organized into 3 armies: the First Army (FROKA), Third Army (TROKA) and Second Operations Command. Each with its own headquarters, corps, and divisions.