Is the third divorce considered valid, making the woman unlawful to her husband, given that she was divorced during the waiting period (iddah) of the second divorce without intercourse, and she has been without menstruation for three years?

If a husband divorces his wife with the third pronouncement while she is still in her waiting period (iddah) from the second pronouncement, then the triple divorce has occurred, and she becomes unlawful for him. She does not become lawful for him again until she marries another husband with a valid marriage (a marriage of desire, not a marriage of tahlil [legalizing her return to her first husband]) and he then divorces her after consummating the marriage.

As for her waiting period if her menstruation has ceased for three years, her waiting period is a full year if she does not menstruate during it. If she menstruates once during that year, she waits for a second menstruation, or a full year without menstruation passes, and so on until her waiting period ends.