[narfforc]

I think the reason that the Elves thought there may be a chance for the Three retaining their power, is because Sauron had nothing to do with their making, he never sullied them, although they could be controlled by the One Ring, the hope of the Elves was that the Three would become free with the the destruction of the One, whereas the Nazgul were totally dependent on the existance of the One Ring, and like all the works that had been done with the power of the One, they were doomed the minute it went into the flames.