upgrading from 2.1.3 to 3.x

If I am upgrading to 3.x from 2.1.3 do I need to upgrade all the minor versions first? 

Or can I upgrade to 3.x directly? There is no documentation on how to do this.

There are instructions:
https://www.codeigniter.com/user_guide/i...e_300.html

there are not many breaking changes - its mainly doing search / replace for renaming things.
suggest- take one part of your application and upgrade it to 3. that will show you what kind of search/replace routines you will need.
and there are system helpers that can automate upper casing the application file names.

My current version is 2.1.3 not 2.2 so does this still apply?

My current version is 2.1.3 not 2.2 so does this still apply?

Yup,  just start with 2.1.3 rather than 2.2