A lv3 boss monster chest is an item in Evony TKR (The King's Return). It can be found after killing certain monsters or bosses, with some of them listed below.
The description of Lv3 Boss Monster Chest in Evony TKR says:
Open the package to gain rare items.
Lv3 Boss Monster Chest is known to drop from a number of bosses or monsters upon killing, including Centaur.
Note: This list may be incomplete.
Lv3 Boss Monster Chest opens up to all of the following items: