You're totally not getting it then...
Any given armor piece counts as its proficiency.
Certain chest plates count as 1 rank lighter when worn alone.
For instance:
Wear a Plate Armor Chest - this counts as a Breastplate in the 'armor suit' list, and as such is medium armor, despite being heavy chestpiece.
Wear a Leather Chest and a Plate Legs - Light chest and Heavy legs. If you do not have Heavy proficiency but you do have Light proficiency, then you take double armor penalties for relevant parts. In this case, your Leather Chest gives +0 armor, +6 max dex, and 0 armor penalty, while Plate Legs nets you +1 armor, +1 max dex, and -3 armor check. So you sum up the armor, +0 and +1 gives you an AC of 1. You then take the worst dex and armor check, +1 dex is the worst on the plate legs, and -3 on skill checks, and because you're not proficient you take -3 to attack rolls as well as skill checks. Thus, this is a terrible suit of armor.
In other words, you sum up your AC and add +1 if you've got all 3 pieces, then you take the worst of your dex and the worst of your armor checks, and compile it into a single suit of armor.
This also means that a Breastplate can get +1 AC if you add Padded Arms and Padded Legs for 1gp each.
All your armor accumulates into 1 solid AC score, and that you have 3 pieces. When you buy 'Plate Arms' it comes with BOTH arms. In fact, vanilla armor suits are made piecemeal but already compiled. When you buy anything off the armor list, it's already had the pieces put together, or you could buy them individually and build an identical suit with identical results (Plate Body, Plate Legs, and Plate Arms ends up with the exact same stats as wearing store-bought Full Plate).