The last option is not corredctky punctuated because when you are using an adverd to conjoin two sentences or pharses you need to ask your self if the two phrases can be by themselves and make sense. So the last one would be you are going to fall off a cliff and you move back. The second sentence does not make sense so you do not need a comma there. When trying to figure this out remember that if they can stand alone use a comma if not do not put one in the sentence.
