Hi Valtorei,
Welcome to our community, and thank you for your questions! 
I want to focus on 3D but I was advised to start with 2D
Do what you feel is best for you. In my personal opinion, it’s always good to know a bit of everything so you know how to ask questions in regards to specific topics and how to find solutions. If you know the typical terms, you will very likely find a solution. Even if you plan to focus on 3D, 2D could still be interesting for you in case you want to create a few icons for your GUI or to edit images.
The game kit course could be interesting if you are not that interested in coding or if you plan to use a game kit for your own game. If you already purchased that course, I would suggest to watch a few videos to see if the content is relevant for you.
Long story short, design your own journey according to your needs and your interests. There is no right and wrong in this respect as long as you are happy with what you are doing.
See also:
- Forum User Guides : How to mark a topic as solved