OK, I'll try and answer some questions without giving too much away.
No official release date yet and although it was hoped to be early September that date has obviously passed by but I think it could be by the end of the month or early October but again, no date has been given yet so this is just my feeling. No software is ever released bug free are there are known issues so I'm guessing they will be putting out some updates shortly after release.
They have a new compression format that is amazing, the original core library was 160GB but with the new compression it is now 54GB with no loss of quality whatsoever.
I've been running it along side BFD2 and Eco and whilst you can load all your old kits in BFD3 you wont be able to load the new content in BFD2 or Eco.
If you have BFD2 then you have all the Eco kits anyway so you wouldn't need to use any of your Eco stuff with BFD3. Also you won't be able to load any BFD Eco presets in BFD3 but you will be able to load all your BFD2 presets and kits.
As for the 16 slots I'm not quite sure what the maximum is but it's a lot more than 16 (BFD2 was 32) that's just that particular kit in the picture that has 16.
Hope that's helped.
Steve