Por que Sauron não controlou os três anéis antes da Grande Guerra?

Porque os elfos que os possuíram tiraram-nos

De O Silmarillion :

But the Elves were not so lightly to be caught. As soon as Sauron set the One Ring upon his finger they were aware of him; and they knew him, and perceived that he would be master of them, and of all that they wrought. Then in anger and fear they took off their rings.

^{The Silmarillion V Of the Rings of Power and the Third Age}

Já que eles não estavam mais usando seus anéis, Sauron não tinha nenhuma habilidade para controlá-los. Uma vez que ele perdeu o Um, sua habilidade de comandar qualquer dos Anéis de Poder foi perdida ¹ ; na verdade, há evidência para sugiro que ele só era capaz de controlar o Nazgûl porque ele tinha os Nove em sua posse física.

¹ Daí porque os elfos consideraram seguro colocá-los de volta e começar a usá-los

Os Três Anéis Élficos não foram feitos por Sauron e não estavam sob seu controle.

Como Gandalf diz a Frodo em Bag End:

The Enemy still lacks one thing to give him strength and knowledge to beat down all resistance, break the last defences, and cover all the lands in a second darkness. He lacks the One Ring.

The Three, fairest of all, the Elf-lords hid from him, and his hand never touched them or sullied them. Seven the Dwarf-kings possessed, but three he has recovered, and the others the dragons have consumed. Nine he gave to Mortal Men, proud and great, and so ensnared them. Long ago they fell under the dominion of the One, and they became Ringwraiths, shadows under his great Shadow, his most terrible servants.

_{The Fellowship of the Ring: The Shadow of the Past}

So it is now: the Nine he has gathered to himself; the Seven also, or else they are destroyed. The Three are hidden still. But that no longer troubles him. He only needs the One; for he made that Ring himself, it is his, and he let a great part of his own former power pass into it, so that he could rule all the others. If he recovers it, then he will command them all again, wherever they be, even the Three, and all that has been wrought with them will be laid bare, and he will be stronger than ever.

_{The Fellowship of the Ring: The Shadow of the Past}