Similar enough to Dune 2 that the learning curve is minimal.
Different enough that you can make sounds with this you literally couldn't make with Dune 2.
Biggest differences, IMO
Additional ARP and functionality
Even without the new filters and FX, the above alone greatly expands the capabilities of the synth.
In addition to that, I think the overall sound quality has improved. Dune 2 had a more "plastic" almost Tone 2 kind of sound to it though not quite as pronounced. Dune 3 appears more neutral.
Also, so far anyway, it appears to be rock solid. I have yet to run into any bugs and I'm running it on a DAW version that isn't even supported anymore so I doubt they even tested for it.
All in all, I'm impressed and pleased.
Is it what I would call a "true" 3.0 as 2 was to 1? No. But it gives me enough in addition that, for me anyway, the upgrade price is acceptable. Please note I said "acceptable" and not "Holy cow, what a great deal" because it's not. Not considering updates made for other synths that were free or for a lot less money.
Still, I'm content. There's enough here that I never have to open Dune 2 ever again, which was, at the time, my favorite synth.
Dune 3 only improves on it.
Enough for you? Don't know. Only you can make that decision.