"Three rings that the Dwarf-sires possessed of old shall be returned to you"
 

It seems clear that the three Sauron's messenger promised to Dain are the three remaining of the Seven Rings: Thror's, plus the two others that didn't get eaten by dragons. I've never thought about this before, but I immediately have two questions:

1. Would he have kept his word? Gloin says that "we knew already that the power that has re-entered Mordor has not changed, and ever it betrayed us of old," but last time round Sauron did hand out every Ring he had possession of apart from the One itself. It's a pretty impressive bribe - a Great Ring in the hands of a dwarf is the foundation of a hoard of gold - and Tolkien repeatedly said that the Dwarves other than Durin's Folk are historically willing to serve Morgoth and Sauron at times, so it's not impossible that three Rings could make them switch sides permanently. But on the other hand... it's Sauron. :D

2. Sauron took possession of "the last of the Seven" around 2845; he offered it back around 3017. He had possession of three unassigned Great Rings for nearly 200 years, and may have had two of them for a couple of thousand years. What, exactly, was he doing with them for all that time? The Seven and the Nine were identical in their creation, so we know that the remaining Seven could be used to create new Nazgul. But they... weren't?

One possibility I've seen before would be that the Mouth of Sauron had one of the rings. Another candidate would be Herumor, the never-written villain of The New Shadow. But in either of those cases, there would be no possibility of turning them over to Dain - and, dwarves being dwarves, you'd think they'd insist on seeing the Rings before they turned over any information.

hS

If that were the case, he would be a wraith

Eventually, yes. Perhaps events simply got in the way before he got that far. Gollum lasted 500 years without becoming even slightly transparent, after all.

hS

Well, true, but Gollum was a hobbit (more or less), and "remarkably tough in the fibre". Based on Appendix B, if some at least of the Nine were "high Numenorean lords," then they would have taken something less than 400 years to become "wraithed."

It is true that the Mouth had not been in the service of the Dark Tower for that long; only since its rebuilding a few decades prior.

Of course, the Nine Rings were being used at the time,* and so Sauron only had three Dwarf-rings at his disposal, the ones his messenger offered to Dain. But I can't help but suspect a plot-hole here; Tolkien getting carried away with the idea that these were specifically "Dwarf-rings" when actually they were all intended for Elves and not for mortals at all.

*By which I mean not that the RW had them, but that Sauron only controlled the Nine so long as he held their rings.

When Sauron created the Nazgûl by leting the 9 chosen Men wield the Rings he had dedicated to them, he himself held the One. At the same time he had tried some similar trick with 7 Dwarves, but in the case of the Dwarves without such effect as Sauron had hoped for: He could not controll the Dwarves with these rings as he could controll the Nazgûl and the livespan of the Dwarves was unchanged and thus they never became wrights. But the enhanced greed and haughtines even hubris of the Dwarvish wielders had for sure some effects in the history that Sauron might have seen as positiv for his propose.

That Sauron saw the 'experiment' with the Rings given to the Dwarves as a failure is seen in the fact that he recollected the 3 Rings that were left to himself in the Third Age. At that stage he no longer possed the One. To have full controll over the Nazgûl Sauron took the 9 Rings from them and wielded these himself. I don't think without the One he could creat new Ringwrights with the 3 remaining 'Dwarf-Rings'. I suppose it was rather for the enhancement of his own abbillity by what ever these Rings offered to their wielder that he recollected them from the Dwarves in the first place.

The offer to Dain (if intended to realise at all) was, in my oppinion, a bid for the future: With the informtion obtained Sauron hoped to find the One, and probabaly he sought he could withhold the 3 Rings long enough to have gained possesion of the One before Dain obtained the 3 Rings. Given the effects seen before, I think, that would not have delivered the Dwarves to Saurons influence at once, but it would have weekend their resistance.

Respectfuly
Findegil