Im curious about how exactly you attain the quests for each piece of armor, look around on the forums and random topics on the offical wow forums aswell but couldnt find any direct info on this so id appreciate an answer from someone who knows. thanks.

Complete Echoes of War.

After you do the Naxxaramas attunement quest, you can find a quest called Echoes of War, which is to kill trash in the zone. This will open up the quests for your Tier 3 set as well as for your Frost Resist Leggings. The last part's just what I've heard, though, I can't personally confirm it, still have one or two more mobs to kill.