According to the official Windows Phone blog a third update has just been pushed out for the Windows Phone 8.1 Developer Preview program, this update listed as build 12397 should be available to download soon. However there aren’t any specific feature changes or fixes in this update, as the WP blog only mentioned that it contains minor tweaks and API changes:
his update enables some newly commercialized devices and tweaks a couple of new APIs, so you won’t see a difference on your device, but we encourage everyone who’s on the Preview for Developers to stay current on the latest build.
Nonetheless, it’s best to stay updated, so head over to your settings and check for updates.
Thanks for the tip Alvester