In the previous mission, Forerunner, Master Chief had his first encounter with the Didact, and soon after that, we had to use a Ghost to escape from the planets core.
Failing to warn Infinity about the gravity well, Halo 4 Infinity starts with a cinematic showing the ship crashing and not long after this, we see the Didacts ship heading the same way. We gain control over Master Chief after the second cutscene, when we arrive at the crash site.
Our first objective in Halo 4 Infinity is to locate the UNSC Forces using their IFF tags and then assist them.
Therefore, we dont have to search for the third Halo 4 Terminal until we find the first group of UNSC soldiers.
Following the Didact to Infinitys crash site, Chief and Cortana descend into the surrounding swamp to rescue UNSC survivors.
RALLY POINT ALPHA
A UNSC teams Friend-Or-Foe tags draw the Master Chief deep into the swamps around the grounded Infinity.
RALLY POINT BRAVO
Reunited with the UNSC, the Master Chief fights to secure a landing zone to evac Cmdr. Laskys team back to Infinity.
RALLY POINT CHARLIE
The push to take back Infinity from the Didact begins as the Master Chief links up with a team of Spartans IVs.
Halo 4 Infinity Terminal Location
The third Halo 4 Terminal is very easy to find, and it can be accessed in the fourth mission, Infinity, right after we start the second part.
The first part of this mission ends with a cutscene where we meet with some UNSC soldiers. After the cutscene, we have to go ahead with two other soldiers.
Instead of following them, we turn around and the Halo 4 Terminal is just in front of us.
After we download the terminal, we complete our tasks in this mission so we can start the next one in line, named Reclaimer.