The stories say there were five once, used to lure the evil dragons of Krynn to their deaths. Now, only three remain. Will you use them to empower and command the dragons... or will they not wheel in CLB draft and now you really need more sources of mana?
They don't actually fix mana since they only produce the color you already needed to cast them, so these will probably end up in the decks that need them pretty easily. Paying 3 to ramp by 1 isn't that great otherwise.