Is swearing by "Divorce be upon me if I do such-and-such" three times, with the intention of emphasizing the oath, and under severe pressure after that action, considered an oath for which expiation is due, as Sheikh Atiya Saqr mentioned, or is it a divorce that takes effect upon my wife?

If you pronounced a triple divorce upon your wife in a single session, contingent on an action, and then you broke your oath: if your intention was to emphasize and strengthen the oath, then only one divorce is binding upon you, unlike if your intention was the multiplicity of divorces in the event of breaking the oath. Divorce is in the hands of the husband and occurs simply by uttering it; the absence of the wife or her lack of knowledge does not prevent its occurrence. As for the view that an expiation for an oath (kaffarat yamin (definition)) is required, this is the opinion of Sheikh al-Islam Ibn Taymiyyah, if you did not intend divorce. And if the pressure you were subjected to does not reach the stage of legal duress, then it does not prevent the occurrence of divorce; but if it was legal duress, then nothing is binding upon you.