[Nerwen]

I lived in a land without hate -Valinor.
With my apprentices three
But by the hands of fate
All three were taken from me

The first one, second in command
Of the Enemy, Marrer of lands -describes Sauron.
I am one of those who guard the light
Even if it's him I'd have to fight -confirms speaker as a good character, therefore not Morgoth, therefore the only other person to whom Sauron was an "apprentice": Aulë.

The second one, overtaken by greed
He dwelt high up in his tower
The destroyer of my beloved's creation
All because he sought power -Saruman, also a Maia of Aulë, lived in Orthanc and his extensive tree-felling can't have endeared him to Yavanna.

The third one, greatest of them all
With skill beyond measure
Great was his fall
Blessed was his treasure -Feanor, whose "blessed treasure" was of course the Silmarils.