The Three Spires is the name given to the Adamantine deposits found below Spearbreakers.
All were found just above the "magma sea" below Spearbreakers. Two were discovered fairly close together down by The Forges during Draignean's reign as Overseer, with the third being found (or possibly simply rediscovered, as much information is always lost during overseer shifts due mostly to typical dwarven incompetance and/or possible sobriety and stress) when Dauros threw a small Core Scout into a large "vent" in the caverns at the request and payment of Mr Frog.
Very little remains of The Three Spires, as once the Shadow Devils were defeated and the Iron Palace was completed all three were mined nearly in thier entirity, with the only remains left unmined being those holding back torrents of molten rock or water. Much of this would go to the arming and armoring of the Army of Spearbreakers, with the rest being slowly processed and kept for later use.