UPDATE: 12th June 2019 – Yes, at some point there will be a 3rd and final Genesis City auction. Several Decentraland districts which did not meet Decentraland’s minimum requirements were dissolved (more info here). These districts were mostly placed in premium locations around the Decentraland map. To date, there is no official schedule released for the 3rd auction.
Maybe. After the second LAND auction, there will be no unowned LAND left. Unless more LAND is made available in Genesis City through dissolving districts (who failed to meet the minimum requirements set by Decentraland), then there will be no more LAND left to auction.
Looking to the more distant future, when Decentraland attracts a large number regular users, it is possible that an additional city may be built. In this case, then it is likely that there will be another auction for the new city LAND to be sold.