I’m using an OpenHab 1.6 in my KNX installation using KNX/IP access gateway. I want to migrate it to the latest OH 3. So, I setup an openhabian with KNX Binding which works without a problem. The “KNX Things” status is Online. As I’m an advanced OH 1.6 user but a newbie at OH 3 I’m totally lost. None of the documentation and examples match the software and possible configuration. I assume it is due to the huge gap between OH1.6 and OH3 and a lot of things has changed in the mean time. So I’m looking for some hints or explanations how to migrate the current “default.items” configuration file to the “things”, “devices” and “items” of the latest and greatest OH3.
Indeed. OH 2 introduced things (I assume you’ve created your KNX gateway thing via UI).
All items now can be created via UI or still as .items files but format has changed, of course.
Check out the OH 2(+3) docs of the KNX binding here.
There’ll be many more differences to show up depending on your setup.
I suggest you check out the 1.X migration tutorial and read through threads about 1.X → 2.X migration as that’s the heavy changes.
Yes, the KNX binding definition in UI was a straightforward and very easy task. 
Thanks. Is there any difference for both kind of configurations (UI vs. file based)? And what should be “preferred” and “best practice”?
I’m reading the pages you pointed. Appreciated.
Yes, see (at least) Configuration | openHAB .
You should read (a lot) on openHAB 2/3 before you proceed.