It is bida’h and unlawful to give three talaqs at the same time. Someone gave three talaqs during the lifetime of the Holy Prophet Muhammad (may peace and blessings of Allah be upon him), so he became angry but enforced all of them. Thus though it is against the sunnah to give three talaqs at a time but if the husband does such act that he gives three talaqs together then all the three talaqs shall occur and the wife shall become mughallazah bainah and thus would be completely haram for the husband. Talaq also takes place if given in the state of anger. Usually, one gives talaq in the state of anger. It is useless to repent after giving talaq and come with the excuse of anger. If talaq is proved by the confession of husband or by Shariah testimony then it cannot be denied.
Allah knows Best!
Darul Uloom Deoband