During the UN-MCR War, the 3rd fleet was operating as part of the UNN Jupiter Fleet. Leading the 3rd fleet was the Jupiter fleet's flagship, the UNN Agatha King, under the command of Admiral Souther. The Agatha King was sent to pick up a L-type dropship from the debris field of the Guanshiyin and Fleet Admiral Augusto Nguyễn tight-beamed the Agatha King ordering to treat the occupant of the dropship as a prisoner and that no one was to speak to him. Unknown to Souther and his crew, Nguyễn was a part of the plot to create Protomolecule hybrids. The Agatha King was then diverted to Io and the rest of 3rd fleet instructed to follow and reinforce after an MCRN cruiser, the Hammurabi moved to intercept the UNN dreadnought.
The Io Campaign
- Main article: Io Campaign (TV)
After learning that Nguyễn was involved in illegal weapons testing from Chrisjen Avasarala and realizing that the Hammurabi did wish to engage when they did not open fire on the Agatha King even though the UNN dreadnought was in range of the MCRN cruiser's torpedoes, Souther relieved Nguyễn of command and ordered his arrest before ordering the 3rd fleet to stand down. Nguyễn, however, pulled out his gun and shot Souther in front of the Agatha King's command crew before Souther could transmit evidence of his treason to the other ships in the A.O.
Ordering the UNN fleet to rejoin and prepare for battle, Nguyễn personally fired a torpedo at the UNN Jimenez and destroyed the ship when it failed to comply with his orders. Contrary to his expectations, this attack caused multiple UNN ships to fire on the Agatha King and one another in retaliation, leading to the destruction of several more UNN ships.