The following is from The Encyclopedia of Arda.
Quote
Those of the Rings of Power that Sauron gave to the Dwarves to seduce them to his service. The Dwarves proved too hardy to be lured in this way, though, and the Rings did little more than increase their native lust for gold. By the end of the Third Age, Sauron had recovered three of the Seven Rings to himself, and the other four had been consumed by dragons.
I don't think Tolkien gets any more specificabout the ones "consumed by dragons." Perhaps one of the Dragon's was Smaug, and since he killed one of the dwarven ring-bearers, people assumed that he destroyed the ring as well.
The ring might have been added to Smaug's great collection of treasures, and then found by Gloin after the treasures were recovered by Thorin Oakenshield's company.
Daerk, we could really use your expertise here (please.)