Each of those words may be used freely to imply "nonsense". However, "ерунда" in colloquial speech may also have another meaning - "trifle, nothing". For example, you stumbled and fell down, and someone looks worrying for you: "How is your leg? Is it painful?" - You can proudly reply: "Ничего, это ерунда!" But you cannot use "чушь" in this sense. "Чушь" is "nonsense" and nothing else!