Obviously the 3rd gen ATV cannot run Siri or perform some of the features the 4th gen can.
I asked this question not too long ago on here.The poster replying to me stated they thought the interface would remain the same for 3rd gen ATVs. We can only guess as to what will happen, as with anything Apple, we'll have to see what drops with the release of the new ATV. Apple could surprise us with a slight makeover for 3rd gen users.
The 3rd gen is still for sale at Apple and would hope Apple might freshen the interface for some consistency appearance-wise with tvOS. I have no plans to run out and replace my 2nd or 3rd gen ATVs. One reason I'm reluctant to upgrade is I have both ATVs feeding a TosLink audio output going into both home theater units. The new ATV lacks that feature. Booo.