Final answer:
The third member of the 2nd Triumvirate who was not Antony or Octavius was General Lepidus. They formed this political alliance in 43 BCE and ruled Rome, but the coalition was short-lived as Octavian and Antony eventually pushed Lepidus aside.
Step-by-step explanation:
The third member of the 2nd Triumvirate, not Antony or Octavius, was a general named Lepidus. The Second Triumvirate formed in 43 BCE consisted of Mark Antony, Octavian (who would later be known as Augustus), and Lepidus. This coalition was a political arrangement that effectively ruled Rome following the assassination of Julius Caesar. Lepidus was initially a significant figure within this alliance, but Mark Antony and Octavian eventually marginalized his role. The triumvirate was responsible for defeating Caesar's assassins at the Battle of Philippi and ordering executions of their political opponents. The partnership between these three men did not last forever, particularly between Mark Antony and Octavian, who vied for control of the Roman Empire, leading to the eventual exclusion of Lepidus from power.