Did the divorce occur by an oath (divorce by three) that was uttered in a state of extreme anger, when the speaker's intention was merely intimidation, and is this considered an act of Zihar?

If you utter words of anger and reach the point of losing awareness, then no divorce or tahrim (making something lawful unlawful) occurs. However, if your anger does not cause you to lose awareness, and the wife disobeys the order, then what you intended by the tahrim takes effect upon her, either divorce or an oath. In the case of an oath, you owe an expiation for breaking an oath.