AT 3, 4, 11, and 12 are superior to other armor... because they aren't armor. They are natural animal armor, when part of the animal. As such, they assume a degree of mobility that exceeds what a suit of armor made from animal hide would have. And for 11 and 12, also probably a thickness, especially at joints, that is not practical for a person to wear.
RMSR says, page 289:
Note: Armor types 3, 4, 11, and 12 are animal armors, natural body coverings with no normal armor equivalents. One cannot achieve such an AT without acquiring some enchanted and specially designed armor (e.g., magic fell beast skin).
So, generally speaking people shouldn't be buying them, and if you have introduced a way to do so, they should be priced according to their significant value.