There are 7 Terminals hidden throughout Halo 4's campaign. The third Terminal, titled "Charum Hakkor", is located approximately halfway through Mission 4 "Infinity" (Rally Point: Bravo).
How to find the "Charum Hakkor" Terminal on Mission 4: Infinity:
After meeting up with fellow UNSC soldiers, you'll witness a cutscene. Following the cinematic, simply turn around and you'll see the Terminal inside their Forerunner structure holdout.
Congrats, you unlocked Terminal 3: Charum Hakkor!
All Halo 4 Terminal Locations