What is the ruling on one who said, "If I return to doing such and such, then my wife, so-and-so, is irrevocably divorced by three pronouncements," and then he did it?

The majority of scholars—including the four madhhabs—hold that a conditional divorce takes effect upon the fulfillment of the condition, and this is the most preponderant opinion. Based on this, if you conditioned your wife's triple divorce upon returning to the aforementioned act, and you did return to it—as you stated—then a triple divorce has occurred. Consequently, your wife becomes unlawful for you, and she does not become lawful for you again until she marries another husband through a valid marriage, a marriage of desire and not one of tahlil (making lawful), and then he divorces her after consummating the marriage. Sheikh al-Islam Ibn Taymiyyah said: An expiation for an oath becomes binding upon you if you did not intend divorce, but if you did intend it, then only a single divorce becomes binding, and in this case, you may take her back before her waiting period (iddah) is complete. The preponderant opinion, as mentioned, is that of the majority of scholars.