Hi BatuBoy,
Welcome to our community! 
Learning something is never a waste of time.
If you completed the Unity 2D course and feel familiar with Unity, you could try to start with the RPG course. If you feel like you are missing knowledge, you could enrol in the Unity 3D course, complete it and go back to the RPG course.
Do what you feel is best for you. We all are individual learners, so there is no “right” and “wrong” when it comes to learning.
Did this help?
See also:
- Forum User Guides : How to mark a topic as solved