[Findegil]

Surely there he was not including the Three. Why would rings made by Elves for Elves have the power to extend the wearer's lifespan?

Why would a ring made by Sauron exclusivly for his own use have the power to extend the wearer's lifespan?
But it had this power for sure, as we see in the case of Gollum.

Originally Posted by Letter 131 to Milton Waldman, 1951:

The chief power (of all the rings alike) was the preservation or slowing of decay (i.e. change viewed as a regrettable thing), the preservation of what is desired or loved, or its semblance – that is more or less an Elvish motive.

This is porbabaly the key to understand the effects. With this the prolongation of mortal lifespan is a given ability of all 19 Rings. We also see a hint how it functions: the Rings gives the mind the possibility to preserve the desired or loved body it does dwell in. An ability that the elves had by thier nature, but mortals had not and dwraves could not even get with the help of the rings.
In that way the wraithification of the bears seems to be very similar to elvish fading. If that is 'true' than it might be that a Ringwraith could have choosen to abondan his life and with that escape from Sauron. But as all the Rings seem to have a great adictive potential and clinging to life longer than seems reasonable is an repeating motive in Middle-Earth mrotals this becomes near to impossible once the ring has worked long enough on the mind of its bearer.

Originally Posted by Letter 131 to Milton Waldman, 1951:

The Elves of Eregion made Three supremely beautiful and powerful rings, almost solely of their own imagination, and directed to the preservation of beauty: they did not confer invisibility.

Here we come to the differnces between the Rings. Sauron did influence the nine and the seven much more than the three. So it seems that the invisibility was an ability that Sauron wished to be included. When the elves worked on their own using only the knowledge that they had learned from Sauron and not his direct help, they could eliminat the invisibility effect.

Respectfuly
Findegil