First of all, there is no such thing as Type 3 muscle fibers. From what I've read, it's basically a term created by some guy trying to sell his training program.
Secondly, and much more importantly, forget you've ever heard about different muscle fiber types. That kind of info is only going to confuse you for now. Make sure you're training hard and being consistent with a well-designed program, and you won't really have to worry about training "these muscle fibers" or "those".
Check this article to get a basic idea of how and what to eat:
You're very active, so you need to make sure you're eating plenty of protein, carbs, and fats. You don't need to be cutting and you don't need to try eating like a strict bodybuilder. You're an athlete and a growing dude, so you need to feed your body like one.
Also, and this is another important point, forget about being an "ectomorph", or wishing you were built like a "mesomorph", or whatever. You're so young and naturally undeveloped, for now, that it's not an accurate guideline and even then, it's a very highly debated method of categorization anyhow. 99% of people are a "mix" anyway, so it's simply inaccurate and is just as easily ignored without an issue.