Praise be to Allah.
If a Muslim marries a Christian woman, then divorces her three times, then she marries a Christian man after that in a valid marriage, and he is intimate with her in the manner in which a man is intimate with his wife, then he divorces her, in that case she becomes permissible for her first husband, the Muslim, after the end of her ‘iddah. That is because of the general meaning of the verse in which Allah, may He be exalted, says (interpretation of the meaning): “And if he has divorced her (the third time), then she is not lawful unto him thereafter until she has married another husband. Then, if the other husband divorces her, it is no sin on both of them that they reunite, provided they feel that they can keep the limits ordained by Allah” [al-Baqarah 2:230]. And the Christian man in this scenario is a husband. This is the view of the majority of scholars.
Imam ash-Shaafa‘i (may Allah have mercy on him) said in al-Umm (4/289):
If a Muslim man divorces his Christian wife three times, then a Christian or a slave marries her, and has intercourse with her, then she becomes permissible to the first husband if the second husband divorces her, once her ‘iddah has ended, because either of these two men is a husband, and Allah, may He be glorified and exalted, says (interpretation of the meaning): “until she has married another husband” [al-Baqarah 2:230]. So in this case she has married another husband. End quote.
Imam Ahmad (may Allah have mercy on him) was asked about a man who married a Christian woman, then divorced her, then she married a Christian man who then divorced her. Can she go back to the Muslim? He said: Yes; do you not see that Allah says (interpretation of the meaning): “until she has married another husband” [al-Baqarah 2:230]? And is this man not a husband? End quote.
And he was asked about a Christian woman who was married to a Muslim, who divorced her three times, and her ‘iddah ended, then she married a Christian man who consummated the marriage with her, then he died or divorced her; is it permissible for her Muslim ex-husband to marry this Christian woman?
He said: Yes, because he was a husband. A Christian may make a dhimmi woman permissible for a Muslim to remarry.
End quote from Ahkaam Ahl al-Milal wa’r-Riddah (p.170)
It says in al-Fataawa al-Hindiyyah (1/473):
If a Christian woman was married to a Muslim and he divorced her three times, then she married a Christian man and he consummated the marriage with her, she becomes permissible for the Muslim who previously divorced her three times (to remarry). End quote.
It says in al-Mawsoo‘ah al-Fiqhiyyah (10/256):
The majority of fuqaha’ are of the view that intercourse with her dhimmi husband makes it permissible for her to remarry the first husband (if the second husband then divorces her), because the Christian man is a husband. But that is contrary to the view of Maalik, Rabee‘ah and Ibn al-Qaasim, who said that this does not make her permissile (for the Muslim man to remarry). End quote.
And Allah knows best.