From Halopedia, the Halo wiki
Terminal 7 is found at the beginning of the final level in Halo 3. To find it, head forward from the start of the level through the snow until you encounter the area with a few structural beams going from one wall into another. At this point (before you go past any beams) you will see a metal floor. Follow the floor to the right and up through a small gap in the ice, and follow this path along. You will find a beam heading through a crack in the ice. Follow this beam into a doorway. Cortana will ask you, “Where are you going?”. Turn left once inside and the Terminal should be in full view. Cortana will say, "Wait. What's that?" as a good hint. The Terminal will be in front of what is presumably one of Installation 04 (II)'s pulse generators. After that, Cortana will ask you: "How many of these have you found?" If the level is completed on Legendary, the Terminal, once accessed on a different difficulty or another run-through, will show a communique from Mendicant Bias himself.
(Upon initial access)
(Upon shutdown, from Mendicant Bias: "I'll tell you who I am. I am Mendicant Bias. This is what I have done.")
(The following is in yellow text on a normal page like the other information)
(Upon being rerouted to a new destination within Terminal)
(The following appears only if you access the terminal on Legendary difficulty)