Do I need to have downloaded the Cerberus Armor in Mass Effect 2 in order to buy it in Mass Effect 3?
Solution 1:
Cerberus Armor is not DLC in Mass Effect 3 - you can purchase it from merchants in the Citadel as soon as you have access to the Presidium. It's one of several complete armor "sets" available.
I never owned any of the DLC in Mass Effect 2, (and I didn't have access to the Cerberus Network) but I was still able to purchase this armor in Mass Effect 3.