Many of the most commonly used get/set methods have been replaced by direct member variables. For example, to set a Node2D's position use:
position = Vector2(x, y)
There are a very large number of changes from 2.1 to 3.0. The API docs and autocomplete are your best friends for finding this stuff out. Not to self-promote, but I did make a video summarizing some of the biggest changes: https://youtu.be/vCUKwT_jb9Q
You can also take a look at this: https://github.com/dploeger/godot-migrationnotes
More guides are on the way. A lot writers had to wait until the release, so they could be accurate with the final versions of things.