会, 能, 可以 — three kinds of can

규칙 会 = knows how, because they learnt it: 我会说汉语。 Also likelihood: 明天会下雨。
能 = able to in these circumstances: 我今天有事，不能去。
可以 = allowed to: 老师，我可以进来吗？
Not allowed is usually 不能 rather than 不可以.