The third update is available who people who are under the Windows Phone 8.1 Developer Preview Program. The new update will bring you to version 12397 from 12393 (second update). This update is only for those who are rocking the developer preview. If you are using newer devices like Lumia 630 you will not get any updates for now as the software is already official.
Again, the details of the update is not given and it states that it will enable new devices and tweaks some APIs. To be honest I have not used my Lumia 620 after the last update as I am using the Mi3 I posted earlier this week. Hope that these updates will improve my experience after I get back to it.
Its always a good thing to received updates frequently but I do hope the official version of WP8.1 and Nokia Cyan update will arrive as soon as possible.