Learning with CI2 Tutorial, but using CI3?

Basically, if you follow a CI 2 tutorial when using CI 3, you're going to have to apply the changes in the upgrade guide. If you can't be bothered to read the upgrade guide to find out what those changes are, then you probably won't be able to follow a CI 2 tutorial and get it to work in CI 3.

To answer the more general question, there are libraries and add-ins available which have fairly simple instructions to get them to work under both versions. Still, some code (primarily code which digs into the internals of CI) requires a lot more work to upgrade from CI 2 to 3.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide