The wording on this trophy/achievement is a little confusing, as one might think you need to max out the levels of everyone’s armors. However, that is not the case, as it pertains to the Armor Shop, a shop you unlock automatically during Main Story 6: Trouble in the Quarry. Upgrading your armors at the Armor Shop will unlock new abilities for your characters, such as a double jump for CJ, or a parry for Garoo. These upgrades can be very important in getting around the various maps, so it’s a good idea to focus more on these than weapons, at the beginning.
As you progress through the game, you will have more quests from the Armor Shop, allowing you to upgrade them further. Note that these upgrades at the Armor Shop are more expensive than the ordinary ones at the Smithy. Once you see the MAX upgrade, that will be the final one. Get the MAX upgrade for all three characters to unlock this trophy/achievement.