Sauron knew of the Three Rings and demanded them back, laying waste to Eregion in his attempt to get them (even killing poor Celebrimbor and using his body as a standard! An ignoble end for the last descendant of Fëanor...I won't go there!). The Elves hid them and did not use them while Sauron retained the Ruling Ring. They were unsullied (would not corrupt their wearers-my note here), having been made by Celebrimbor without Sauron's influence, and yet they were subject to the One Ring. When Sauron lost the One Ring, the Elves could use their 3 Rings, but if Sauron regained the One Ring while the Elves wore the 3, Sauron would instantly be aware of them. His Ring rules them all, after all!
I have often wondered if this is why Frodo can hear Gandalf's voice on Amon Hen when he is wearing the Ring. "Take off the Ring, fool!" The info about the 3 Rings I posted above is from "Of the Rings of Power and the Third Age" section of my copy of the Silmarillion. Hope this helps!
Cheers,
Lyta
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“…she laid herself to rest upon Cerin Amroth; and there is her green grave, until the world is changed, and all the days of her life are utterly forgotten by men that come after, and elanor and niphredil bloom no more east of the Sea.”
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