Can a couple return to each other after the third divorce, given that the wife was divorced three separate times, and in both the first and second divorces, her husband took her back during the iddah (waiting period) and they had intercourse, and that her menstrual cycle had stopped before the first divorce?

Since the husband divorced his wife with three separate pronouncements of divorce, she has become irrevocably separated from him (baynunah kubra). She is not lawful for him again unless she marries another husband – a marriage of desire, not a tahlil marriage (marriage for the purpose of making her lawful for the first husband) – and the new husband consummates the marriage with her, and then he divorces her or dies, and her waiting period (iddah) from him ends. As for a woman who has despaired of menstruation (i.e., is post-menopausal), her divorce is valid at any time; there is no specific sunnah or bid'ah (innovation) regarding the timing of her divorce.