I'm going to high school. What level can I achieve by studying China painting and calligraphy for three years?

Well, it's personal. You must choose the right one. At first, I chose calligraphy (writing brush). I just got the Grade 8 certificate yesterday and studied for about two and a half years. Moreover, Chinese painting and calligraphy are a little similar, a little basic, and will learn a lot, all based on pens. I hope my answer is helpful to you.