The Three Keys to Power are the magical Grimorum Arcanorum book, the Eye of Odin, and the Phoenix Gate. When united or brought together, they can turn their bearer into an semi-invulnerable sorcerer or sorceress of immeasurable magical power, strength and abilities of varying intensities.
By 1975, the Archmage was already coveting the Three Keys; he had the Grimorum in his possession by this time, and sought to steal the Phoenix Gate as the next step to achieving his goal, though without success.
Brother Valmont was also aware of the Three Keys to Power, and desired them as well. He took the first step towards this goal when he seized the Grimorum book, at the Battle of Rathveramoen, but was subsequently defeated by Brooklyn, losing the large magic book to him.
By various paths, the Grimorum, the Eye, and the Phoenix Gate all found their way into Goliath's keeping by 1995. He stored them in the clock tower until Demona and Macbeth, under the gas of the Weird Sisters stole the Three Keys later that year. They and the Weird Sisters then delivered the Three Keys to the Archmage, who united them by donning the Eye of Odin upon his skullcap, devouring the Grimorum Arcanoum book (in order to provide a loophole that would allow him to enter Avalon itself with it), and wore the mighty Phoenix Gate upon his chest. With these three objects combined, in addition to his own strong magic, the Archmage was transformed into so great, so mighty a sorcerer that he could even use the Phoenix Gate's immensely powerful abilities without having to speak the Latin incantations to command it fully, and proceeded to attack Avalon; as his first step towards world conquest.
Goliath finally defeated the Archmage by tearing the Eye of Odin from his brow; once the Archmage lost the Eye, he was unable to control the Grimorum any further, which self-destructed, consuming him in the process. Goliath took the remaining two Keys into his keeping again, eventually returning the Eye to Odin and throwing the Phoenix Gate into the time-stream after Puck tried to trick Goliath into giving the Gate to him (the Gate reappeared in New York in 1997, only to disintegrate, freeing the Phoenix in the process). With two of the Keys thus destroyed, and the Eye restored to Odin himself, it is extremely unlikely that anyone will succeed in finding and uniting the Three Keys of Power ever again.