Because there is another forum (in Russian Language), when some guys, with little help from me, research what type of chests have Gothic 3 and what type of items you can get from them. I try describe it, sorry for language because I don't know English very well. So they found there is 8 types of chests:
1)Chest with white word "Chest" above. Without lock, small number of random loot.
2)Chest with red word "Chest" above. Lock, normal number of random loot
3)Chest with red word "Metal Chest" above. 2nd level Lock, big number of random loot
4)Chest without word above, 1st level Lock, big number of random loot with one permanent weapon item. Loot changed with Save/Load, but 1 item always be here.
5)Chest without word above, 1st level Lock, big number of random loot with one permanent magic item.
6)Chest without word above, 2nd level Lock, with one random high level item (scroll with high level magic, blank magic ore, magic staff or other things) and low level loot.
7)Chest with 3rd level Lock, with permanent number of gold. More open chests of this type - more number of gold
8)Permanent chests (like Alchemist chest in Cape Dun, Ali's Chest in Faring, Ashton's Chest in Montera, tutorial chests in Ardea). Permanent loot.
There is 2 list of items for chests 4-5. #13 is Bearded Axe, #14 is Katana, #15 is Paladin's Sword in weapon list, so you need open 14 chests of this type to get Katana, there is Bearded Axe in 13th chest and Paladin's Sword in 15th chest.