All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If the institution which gave you the Fatwa as to the permissibility of taking you back is a reputable institution, then there is no harm on you – if Allaah wills - in acting according to their view.
Indeed there is a difference of opinion among the scholars on the issue of three divorces in one session, or divorcing when the woman is in menses. The majority of the scholars are of the view that three divorces in one session is valid and divorce when in menses is valid too.
However, Ibn Taymiyyah and his student Ibn Al-Qayyim are of the view that three divorces in one session are considered only one divorce and that divorcing a woman in menses in not valid. This is indeed also a valid opinion, and the matter in both cases is a matter of Ijtihaad (a juristic opinion on matters not specified explicitly in the Quran or the Sunnah).
In this regard, please refer to Fatwas 81864 and 87485.
Finally, it should be noted that the spouses should therefore maintain the precious contract that exists between them and avoid as much as possible causes of dispute.
Allaah Knows best.