What is the religious ruling on saying "triple divorce" in one's heart without uttering it or moving one's lips?

As long as you did not utter this phrase audibly to yourself, it does not result in a conditional or immediate divorce. Divorce does not occur through self-talk. And assuming you did utter it unintentionally, such as a slip of the tongue, or you uttered it while absentminded, astonished, or overwhelmed by obsessive thoughts, then nothing results from it.