How do I get the Marathon Man achievement in Halo 3?

OK, so I've gone through and did "The Ark", I red ALL THE WAY THROUGH the three terminals on that level, I mean I actually sat there and read them until it didn't go any further, even after the screen turns red. I waited until the little arrow turned red. Then I finished the level after getting all three of them. The I did the EXACT same thing for "The Covenant" and "Halo" AND NO ACHIEVEMENT!!!! I did all of them on normal AND in cronological order.