I don't feel much of a durabilty increase when adding a 3rd armor item

{quoted}

Armor has diminishing returns the more you get, yes. The armor formula for % damage taken is 100/100+armor. Just to give you some examples, 100 armor = 50% reduced damage, 200 armor = 66% reduced damage, 300 armor = 75% reduced damage, 400 armor = 80% reduced damage.

http://i.imgur.com/il8VKrM.png

Also the more armor you get, the better become against you.

armor doesnt have diminishing returns. its a common misconception. the % damage reduction values in the tooltip are misleading.

what armor actually does is increase your health pool against physical damage by 1% for each point of armor you buy. the % damage reduction value in the tooltip only compares the effective health off the last point of armor you got to your total effective health pool or some funky shit. of course the more armor you have, the more that number is going to go down.

however, when you have 500 armor, that one point of armor after 499 adds exactly the same amount of effective health to your character as the first point of armor.

however, last whisper straight up cuts the value of every point of armor you buy in half. which usually happens by the time you get your third armor item.

{quoted}

Armor has diminishing returns the more you get, yes. The armor formula for % damage taken is 100/100+armor. Just to give you some examples, 100 armor = 50% reduced damage, 200 armor = 66% reduced damage, 300 armor = 75% reduced damage, 400 armor = 80% reduced damage.

http://i.imgur.com/il8VKrM.png

Also the more armor you get, the better become against you.

Incorrect.

Each armor is an effective 1% more health against physical damage. MR is the same but for magic damage.

the equation is 100/(100+ base + bonus )

Edit: that being said the amount of armor pen/reduction is to damn high.