You will probably have to level Rahool all the way and reset rank. It will open up a fourth focusing tab with him to that allows you to unlock specific exotic armors that you have not done yet. You will need to use normal materials and an exotic cipher. Once your exotic is unlocked - I believe you can then utilize tab 3 to unlock a specific piece for less material costs. I forgot to hoard exotic engrams before the new episode. So I farmed legendary lost sectors for materials and engrams to open up all the focusing tabs.