When upgrading from 2.x, expect a new and refreshed editor.
Most of your configuration will be automatically converted to the new configuration sets, but some features have been dropped or work different now.
It's worthwhile considering to start fresh, rather than trying to recreate an exact 2.x setup, to make the most of what's new. At the very least, take a good tour of the new configuration sets to see what Redactor 3 can do.
Start with the Quickstart to learn what Redactor 3 is all about.
If you're missing features from 2.x, please let us know.