Is The Warrior Sentinal Insignia Stackable

I want to give Koss the best armor I can. He has Primeevil now. My question is if I buy three Warrior Sentinal Insignias which gives +20 Armor (13 Strength Requirement VS Elemental damage) will he have +60 vs elemental damage. It does not say if it Stackable or Non-Stackable. Since they cost 1800 gold I need to know if he will have +60 Armor vs elemental damage or not. In fact are all insignias stackable or not????

No. They won't give you +60 armor.

If a single piece of armour has an AL bonus, then it only applies to that part of the body. For example, if I stick a Blessed insigina on my chest but have the rest as Radiant, I only get the +10AL on my chest.