As far as I know the keys are gotten from three Hell Bosses (Countess, Summoner and erm... dunno).
A full keyset can open up portals to three mini ubers (uber forms of andy and duriel I think and erm... dunno again ) (cube em in Harrogath)
Those mini ubers can drop organs.
A set of organs should allow you to open up a portal to Uber Tristram, where you can fight Uber Meph/Diablo/Baal att the same time. (cube again from Harrogath).
Doing this succesfully can score you torches, standard of heroes and erm... dunno
Third key is Nithlatak.
Third Organ is UberIzual.