Theres one thing I dont understand?
 

In the book "Tolkien companion", it says if Sauron regained the one ring, he would then be able to control the wearers of the 3 elven rings minds and look into there hearts, and all that has been wrought with them laid bare to him- even if they hid them again. So what if before Sauron regained the one Galadrial, Elrond & Gandalf left their rings in Middle-earth and left for the undying lands? What would happen then?

Middle Earth would fall under Sauron's control, whether those three were there at the time, or not.

-Menelien

I don't think that Sauron would have had much use for the Elven Rings, except to bring all of the Elves remaining in Middle-earth under his dominion, because they weren't made "for him." The Gwaith-i-Mirdain crafted those rings for the Elves, with Elven purposes. At best, Sauron would have been able to see into the minds of Elves, but he wouldn't have been able to twist the Rings to his own purposes.

I think location is pretty much irrelevant to rings of power, so I think the effects would be basically the same.

Gandalf, Elrond, and Galadriel would have been considered to be forsaking Middle-earth. Galadriel had been banned, she could not have returned at the time--her ban was only lifted with the refusal of the One Ring and her help of the Ringbearer. Gandalf had been sent from the West to aid Middle-earth. His mission hardly would have been fulfilled if he had just given up and gone back into the West: he perhaps would not have been welcomed back (and i don't think I need that perhaps).

That's disputable, Westerly Wizard. In UT Tolkien presents both this idea, and the possibility that she had set her own ban, and does not clarify the matter.