There are two-parts to this answer: the first about existing user-stacks and the second about creating new ones.
Existing user stacks
Yes. Existing user stacks created in Stacks 2 are in Stacks 3 and appear in your library. They are exactly the same and will continue to work in exactly the same way.
Partials are a new feature of Stacks 3 and are, in many ways, user-stacks with many more features including the ability to save them inside your document and continue to edit them indefinitely.
There is a good video which shows how to use partials here: https://vimeo.com/136154079