A Sindarin word for one of three
that raised their fingers to the sky.
Here within living rock a chamber be.
An endless stair to lead to on high.
First two sentences leads me to think of two possibilities…
Either one of three is one of the three elven lords who got a ring,
or else it’s one of the three races who got a ring (elves, dwarves, men)
Third and fourth, puzzles me…
Chamber of living rock… could that still be dwarves?
Or did one of the elven lords live beneath the rock?
I think the key phrase is ‘endless stair to lead on high’.
The ‘Endless Stair’ was winding walkway that led from the deepest parts of Khazad-dum to Durin’s Tower on the peak of Zirakzigil. That’s where Gandalf finally kills the Balrog of Moria in the Lord of the Rings.
Now Moria was built under three mountains - Caradhras, Celebdil (the Sindarin name for Zirakzigil) and another one that I can’t remember. So I think the answer is Celebdil.