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.
Khul' is a minor irrevocable divorce according to the Hanafi, Shaafi'i, Maaliki, and Hanbali (one narration) Schools of jurisprudence. This is the preponderant opinion of the Muslim scholars.
Therefore, if this woman has already been divorced twice and then she got khul', which is a minor irrevocable divorce, then this is a major irrevocable divorce and she becomes unlawful for her husband. He cannot take her back unless she marries another man a serious marriage and he consummates the marriage with her[has sex with her], and then he divorces her [or dies]. In this case, the previous husband can marry her fulfilling all the conditions of a new marriage contract (and a new dowry).
As for the disagreement of the husband and the wife concerning the numbers of divorce that are delivered, then it is the husband's confession that is considered according to the Hanafi, Shaafi'i, Maaliki, and Hanbali Schools of jurisprudence.
Imaam Ibn Qudaamah wrote in his book entitled Al-Mughni: "If a husband denies the claim of divorce made by his wife, his rejection is accepted because, in principle, the bond of marriage is still valid and there is no divorce, unless she has strong evidence that supports her claim. Evidence here means that two trustworthy men should witness the divorce. In case there are no witnesses, there are two narrations about whether or not the husband should make an oath to prove his truthfulness. Abu Al-Khattaab reported that the husband should take an oath and this is the correct opinion as the Prophet said that the defendant has to make an oath and the person who denies has to make an oath."
Therefore, if a wife is certain that her husband has divorced her thrice while her husband denies that, it is absolutely not allowed for such a woman to allow her husband to enjoy sex with her even though his claim is accepted, as, in this case, it would be Zina (fornication or adultery), even if a judge ruled that she is not divorced from him, because she is not lawful for him at all after the third divorce until she marries another man. So, she should run away from him or ask for Khul' even in return for compensation.
Ibn Al-Humaam said in Fath Al-Qadeer: "The woman is like a judge it is not permissible to let her husband have sexual intercourse with her if she knows from him what appears to be the contrary of what he claims."
Besides, Ibn Nujaym from the Hanafi School, writes in his book entitled Al-Bahr Ar-Raa'iq: "If two men witness in her presence that he [her husband] divorced her, she should not allow him to have sexual intercourse with her."
Al-Bahr Ar-Raa'iq also reads: "The same thing applies if she hears divorce from him, while he is denying it, and the judge issued Khul' against him, and then he [the husband] brought her back to him, it is still not permissible for her to live with him."
Therefore, if you are certain that you divorced your wife only once while she insists that it is two divorces, and then she gets Khul', which is one irrevocable divorce, then, in this case, it is your statement that should be taken into account. So, the Khul' is considered as the second divorce for you and it becomes permissible for you to conduct a new marriage contract with her if she agrees with that.
However, if your wife is certain that you had divorced her twice before Khul', so the Khul' is the third divorce for her; in which case, it is not permissible for her to accept this marriage.
Based on the above, we advise you to reconcile with your wife and solve the problem that made her demand Khul' if this is Islamically and according to the customs possible. Besides, you should try as much as possible not to let her get Khul'. Finally, you should both know that the bond of marriage is a great bond and the stability of the marital life lies in it. Both the husband and the wife should try their best to keep these relations and fulfill the rights of each other and forgive each other's mistakes as much as possible and they should both give priority to the common interests over individual ones.
Allaah Knows best.