Both of the possible options has been claimed in a few URL variants by Mark Monitor, the company who registered the other Hobbit domains for the producers. Fusible spotted the registrations just before the weekend.
The two options are Riddles in the Dark and The Desolation of Smaug.
I feel that the latter would make most sense as the title of a third film, as it refers to the area around Smaug's lair in the Lonely Mountain, somewhere that the Dragon has destroyed and left, well, desolate. This would likely have to be the second of the three films, relative to where the location appears in the overall narrative.
Less literally appropriate to a third film would be Riddles in the Dark, the name of the fifth chapter in the book. These scenes take place during the narrative span of The Unexpected Journey, at least as it stands, and a sequence from this chapter was played, in at least nearly-finished form at Comic-Con this month.
So, given this, it's possible that these aren't potential names for the third film. Perhaps they'll be part of a marketing viral; perhaps they'll be the names of some kind of ancillary product – video games maybe; or perhaps the tiny expenditure needed to claim these URLs was spent pre-emptively to scare off cyber squatters.
They are going to need a title for the third film at some point. For now, I'm going to gamble on The Desolation of Smaug. Fits well, sounds dramatic, and leaves The Battle of Five Armies to take centre stage in the third picture.