I found the key#3 in the bookshelf of a “workshop” (I can’t be sure of its name) in a GT dungeon, but it doesn’t seem to correspond to a certain safe. Generally, the four safes in the “dorm” correspond to keys 1, 2, 4, 5, and I don’t have key#3 in another GT dungeon, so what’s the use of this key? I…

Nah, it’s a Fake Key to throw people off and confuse them. XD