All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If this man uttered an explicit wording of immediate divorce, like saying to his wife, “You are divorced,” then divorce takes place, and his intention is irrelevant in this case because an explicit divorce does not require an intention, and his ignorance of the religious ruling has no effect. He has incurred this upon himself due to his neglect; he should have asked about the rulings he is ignorant of rather than rely on his own understanding of what he read! Allah, The Exalted, Says (what means): {So ask the people of the message if you do not know.} [Quran 16:43]
If he is not certain that this was the third divorce, he should disregard the doubts and act on what he is certain of because, according to the established principle stated by the jurists and evidenced by the authentic Sunnah, certainty is not overruled by doubt. Based on that, he has the right to revoke the divorce and take his wife back in marriage, and she has not been irrevocably divorced from him until it is proven otherwise.
However, if his statement was only a threat of divorce and not an immediate divorce, by saying to her: “I will divorce you,” then in this case divorce does not take place until he carries out the threat by issuing an immediate divorce.
Allah Knows best.