Is it permissible for me to marry my Russian ex-wife again, whom I divorced with a triple तलाक (triple divorce), knowing that she did not marry another man but had an illicit relationship, and she is now ready to embrace Islam? And what is Islam's stance on marrying a non-Muslim woman?

He who divorces his wife thrice may not marry her again until she marries another man and then separates from him after consummating the marriage. Adultery does not make a woman lawful for her former husband; rather, it increases the prohibition. It is impermissible to marry an adulteress until she repents. It is permissible to marry a chaste woman from the People of the Book, though it is better not to, and it is impermissible to marry a non-Kitābī woman, such as an atheist. It is advisable to marry a woman of faith and good character. You may marry the aforementioned woman if she embraces Islam, her condition improves, and she separates from her husband, provided she was from the People of the Book before her conversion to Islam. If she was not from the People of the Book, then your marriage to her was invalid, and the divorce holds no value. In this case, you may marry her after her conversion to Islam without the need for an intermediate husband (muḥallil).