Divorced 3 times for adultery and her first ex-husband wants to remarry her after her second marriage
30-6-2005 | IslamWeb
Question:
I was married to a man and we divorced because I cheated on him. We did divorce three times because he kept taking me back. I then married and had a daughter with this man (He is not Muslim). Now this new husband has left me because I cheated on him. I have recently saw my ex-husband again and he is not married again. He has asked me to marry with him again because even though I cheated on him that he still loves me. Is it legal that we marry again?
Answer:
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 
is His slave and Messenger.
First of all, we invite you to sincerely repent from the sin of Zina (fornication or adultery) which is one of the major and most heinous sins.
If a believer ponders on the texts about the punishment of Zina, he/she will become frightened and writhe with fear. Allaah says (what means): {And those who do not invoke with Allaah another deity or kill the soul which Allaah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated. Except for those who repent, believe and do righteous work. For them Allaah will replace their evil deeds with good. And ever is Allaah Forgiving and Merciful.}[Quran 25:68-70]
Moreover, the Prophet informed us that on the night of Al-Israa' (Ascension) he saw adulterers and adulteresses naked in a hole like an oven with a narrow top and wide bottom, and the fire was kindling underneath that hole. Whenever the fire flared up, the people were lifted up to such an extent that they were about to get out of it, and whenever the fire subsided, the people went back down into it… when he enquired about them, he was told that they were adulterers and adulteresses." [Al-Bukhari]
Also, the Prophet said: "An adulterer [or adulteress] is not a true believer at the time he/she commits adultery." [Al-Bukhari and Muslim]
The sin of Zina is more abominable if the woman who committed it has a husband and the punishment, in this case, is more severe since it leads to many bad consequences such as violating the rights and honour of her husband and placing in his lineage that which is not from him, etc.
Therefore, you are obliged to repent to Allaah before it is too late. You should bear in mind that you will stand in front of Allaah, the Glorified and the Exalted, naked and barefooted in a Day, the duration of which, is fifty thousand years, and at that time, you will feel the repulsiveness of the sin that you had committed many times.
For more benefit, please refer to Fataawa 86631, 84637 and 83904.
As for your question whether you may return to your ex-husband after having married a non-Muslim, then the answer is that you cannot do so for two reasons:
Firstly, your marriage to a non-Muslim man is not considered a valid marriage in Islamic jurisprudence since a Muslim woman is not allowed to marry a non-Muslim and vice-versa, as we mentioned in Fataawa 87151, 88406 and 81220.
If you had married him having the knowledge that this is prohibited, then this is considered an extra sin, so you have to sincerely repent from it. The scholars 
have mentioned explicitly that the woman who has been divorced thrice cannot remarry her previous husband after an invalid marriage as Allaah says (what means): {And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him…}[Quran 2:230]. It means a valid Islamic marriage.
Secondly, it is not permissible to marry an adulteress if she did not repent from having committed Zina even if she is a Muslim. Allaah says (what means): {...and the adulteress none marries her except an adulterer or a polytheist…}[Quran 24:3].
Therefore, it is not permissible for your previous husband or any other Muslim man to marry you until you sincerely repent, observe a waiting period, and then your waiting period expires.
After sincerely repenting and your waiting period expires, it becomes permissible for a Muslim man to marry you. However, your previous husband cannot marry you until you get married to another man a valid marriage and he consummates the marriage with you and then he divorces you or dies.
Allaah Knows best.