[Elfstone]

Well first of all, Tulkas et al aren't Istari; they are Valar.

Obviously. And it's also obvious that Sauron would be hard pressed to even put up a good fight against any of them. But the post said Istari or Tulkas, Eonwe, and Orome. That's why I bolded Istari in the quote, because I am curious if Legolas thinks Gandalf could handle that battle.

And see here: Gandalf didn't exactly lose his life to the Balrog, did he?

I'm not exactly sure what you mean. He obviously didn't lose his immortal being as Olorin the Maia, but Gandalf the Grey, mortal being of Middle Earth, ceased to exist (and was sent back as the more powerful Gandalf the White). And since that happened as a direct result of his battle with the Balrog, I think "lost his life in a battle with a Balrog" is accurate.

But that's neither here nor there. I'm curious as to why anyone thinks that there is an exception for the Three in what Gandalf is saying about the Rings of Power. To me there is nothing there that would give even a slight hint that Gandalf meant to exclude the Three when describing the Great Rings as perilous or saying that no one ever gives them up, or implying that this was the case only for mortals but not for Elves. It appears to me that J.R.R. just slipped up.