Interesting discussion. I, too, like the idea of the Three Rings as representing the past, present and future, though it would have to fitted also to the other levels of symbolism such them also being the rings of fire, water and air. In terms of their effect for preservation, we might say that Elrond's Ring helped him preserve the loreof the Elder Days, Galadriel's Ring allowed her to preserve the feelingof the Elder Days, while Gandalf's Ring allowed him to preserve (or rather to re-create) the spirit(the modto use the Old English word) of the Elder Days.
The main power of all the rings,writes Tolkien in a letter, was preservation. But not a preservation of what was (in the middle of the Second Age), but rather a preservation of what had been in the First Age, and so they also had powers of healing. In this purpose of embalming, the Elves do, in my opinion, resort to magic, properly so called: there is little‘art’in the desire to dominate Middle-earth, or at least parts of it, to the point where all change (i.e. not merely decay) is arrested. As others have already pointed out, the Elves didn't make any of the Rings of Power (not just the Three) to overcome any enemy, unless you count the nature of Middle-earth as their enemy, but this desire to overcome, to dominate the natural order of Middle-earth, is of the Machine rather than of Art (magic, not enchantment).
An interesting tangent to this discussion is the effects of this on Men: decay is arrested and so their physical bodies do not age, but with the Three not conveying invisibility, it is questionable whether they would, in time, have turned a human keeper into a wraith (another question is whether the invisibility effect is really a part of Sauron's perversion of the other Rings).
As for Sauron's ability to control the Three, I think LotR's computer analogy is excellent— Sauron had, so to speak, more or less written the operating system, so it didn't matter if he had no hand in the hardware, for he still had the back-door that he needed to take over control of the Ring.
Sorry— I am rambling a bit, trying to address every point and express every thought that I noticed while reading It is in any case a very interesting thread.
Troels Forchhammer, physicist, Denmark
The love of Faery is the love of love: a relationship toward all things, animate and inanimate, which includes love and respect ...