The 3 Mysterious Keys are used to open the 3 chests on Doom Volcano. The first key is given to you by the guy in barkers town who quizzes you about the town's residents ( I apologize I don't know his actual name right now). The second is given to the player after retrieving Brownbeard's (the dead pirate) bones from Tropical Retreat. The player must talk to the pirate captain after getting the bones. The third key is located in The Settlement. The player must upgrade/buy every building before the key is accessible. The key is by the windmill to the left of the player's second house.