Does a divorce that occurs after a wife threatens her husband with reporting him to his superiors at work take effect, and is it considered a divorce under duress, knowing that two previous divorces have already occurred between them?

The wife's threat to her husband with a complaint is not considered duress in divorce. The recognized duress is that which makes it highly probable that severe harm will occur, such as killing or severe beating. Accordingly, their divorce is valid. If he had divorced her twice before, she has become irrevocably divorced from him with a major separation. They cannot return to each other unless she marries another husband in a valid marriage and he then divorces her after consummating the marriage.