I just ate and now I'm hungry again. Is that normal for a boy who is on stage 3 with puberty?
It is very common. You are growing rapidly, which requires a good deal of energy. The near constant hunger is where the jokes about teenagers eating their parents out of house and home come from.
What you don't want to do is fill up on junk food. Get some fruit or find something high in protein, but avoid things that are heavy in fat. Foods with lots of salt tend to just make you more hungry and thirsty.