Rules are rule. There are a few small breaking changes you’ll have to deal with mostly having to do with Joda DateTime being replaced with ZonedDateTime and executeCommandLine’s arguments changed. Nothing else changed so talking about 2.x and 3.x rules doesn’t really mean anything.
There is the ability to define rules through the MainUI but not even that is new, you could do that in OH 2.x too with the Experimental Rules Engine installed (which is now just The Rules Engine).
If you are asking about whether there is a way to convert rules in a .rules file to UI rules the answer is no. They have some fundamental differences in structure and definition that makes developing a tool to automatically convert them all but impossible. But why convert them? If you are happy with rules in .rules files then stick with rules in .rules files.
That’s one set of libraries. I have my own at GitHub - rkoshak/openhab-rules-tools: Library functions, classes, and examples to reuse in the development of new Rules. though I expect most people would create their own custom libraries most of the time. OH 3 Examples: Writing and using JavaScript Libraries in MainUI created Rules. Note Rules DSL doesn’t support libraries.