Well a Enhanced Edition for the previous games was just the 'final edition'. After the EE, CDPR, should have the game on the market they want, It doens't just fix bugs... It fixes problems with the game, like: content missing (Act 3 quests in TW2), better ui (TW1).
Bugs can always be fixed/patched, the EE just 'finishes' the game content wise. Atleast that's what it has done up untill this point.
And they can only start work on a EE for TW3 after TW3 is released. They might have a list somewhere that they still want to implement, but community feedback is also a big factor of what get's into the EE. So that's why there are no plans for it as of yet. I guess it will also take a while longer then just a year after launch, since the game is a lot bigger then the previous 2 games.
Also, I doubt DX12 would be implemented in the EE, it's quite a big feature that needs to have a engine developed around it. (I think)
I don't think it's something you can 'just' update into the game. Even if it can, save games might not be compatible. (Look at Dark Souls 2 with their dx9 and dx11 version.)