I do not have an answer that I am confident with, but I'll try this one anyways; maybe I'll even go in the right direction. And this could always give other people ideas.
To first of you, I'm just a tale,
told by the skies, as they unveil
mysteries hidden long ago. - Seek for the Sword that was Broken, in Imladris it dwells...
To second one, who's bigger still
than me, I'm first a fear until
I show him I can bring your friends;
his trust shall be of importance. - dunno. Aragorn and Sam, perhaps? Sam was afraid and suspicious of Aragorn at first. But it's Aragorn that Sam dislikes, not his sword. And it is Glorfindel's appearance that puts him at ease, not Anduril's ability to "bring friends"...
And to the third, whom you don't know,
I enlighten the terror's night, - could be a reference to the victory at Dagolrad.
and in mine and the Other's flight
I save more than the tale did show. - Ohtar fled Isildur's death scene with Narsil, thus saving hope and enabling the latter generations (ie Aragorn) to use this powerful weapon again, thus bringing about Sauron's defeat.
No, I don't think it is the Shards of Narsil (or Narsil, or Anduril, or the Sword that was Broken, or what have you), but I am wondering if I am interpreting certain things right. For one thing, I think the first stanza is somehow referring to a vision or dream.
I mean, it would be much easier if the answer could be Dragonglass!
