Bronze Keys and Guardian Keys are only legitimately obtainable through commands or inv-editors.
Dungeon Keys are dropped by the three mini-bosses in Slider's Labyrinth, which are Slider Host Mimic, Sentry Guardian, Labyrinth's Eye. They are not physical items but instead appear on the HUD when obtained. They resemble the unused Guardian Key and three of them are needed to open the entrance to the Slider's room, so all three must be defeated before getting to fight the Slider.
In the original Aether Mod, the color of the key corresponded to the dungeon type (Bronze, Silver and Gold)
Slider Host Mimic and Labyrinth's Eye used to have their own corresponding keys, but later updates removed them in favor of a singular key system.