Is a divorce enacted by my saying to my wife, "If you speak to me incorrectly again, you are divorced," knowing that this is the third divorce?

If you intended by this phrase that divorce would occur if your wife did what you forbade her from, then she is divorced by her action. However, if you did not intend divorce but rather a threat or prevention, then there is a difference of opinion among scholars concerning it. The preponderant view is that divorce occurs when the wife does what she was forbidden from. Sheikh al-Islam Ibn Taymiyyah, however, holds the view that it does not occur, and an expiation for breaking an oath (kaffarat yamin (definition)) is required. It is advisable to present the matter to local scholars. One must be wary of anger, for it is the key to evil.