بسم الله الرحمن الرحيم
It goes without any doubt that as per the Islamic Shariah the sunnah method of giving talaq is that three talaqs are given in three tuhr (menstruation free period). But if someone gave three talaqs in single meeting against sunnah then also all the three talaqs shall take place, but the person who gave talaq shall be sinful due to his non-sunnah practice. There is a Sahih hadith in Abu Dawood:
فطلقها ثلاثا عند رسول الله صلى الله عليه وسلم فانفذه رسول الله صلى الله عليه وسلم .
Allama Shaukani has categorized it as Sahih Hadith. As per this hadith a Sahabi gave his wife three talaqs in single meeting and the holy Prophet Muhammad (صلى الله عليه وسلم) enforced all the three talaqs. Hadhrat Imam Bukhari (رحمة الله) also has the same view. This issue is agreed upon by the four Imams i.e. Hadhrat Imam Abu Hanifa, Imam Malik, Imam Shafai and Imam Ahmad ibn Hambal (رحمهم الله). There is not a single hadith in which the holy Prophet Muhammad (صلى الله عليه وسلم) declared three talaqs as one talaq nor he ordered for ruju. It is not right to produce the saying of Hadhrat Ib Abbas (رضى الله) as the saying of Sahabi is not an evidence as per Ahl Hadith people. Moreover, Hadhrat Ibn Abbas (رضى الله) used to issue Fatwa against his riwayat, whereas it is a principle of Hadith that if a narrator of any hadith gives fatwa against his riwayat then his riwayat is unacceptable rather his fatwa shall be accepted. One conference was held on the above mentioned issue in Saudi Arabia which passed unanimously the resolution that the three talaqs of one meeting shall be ruled as three. One booklet also was published from there; you may study it (http://www.darululoom-deoband.com/urdu/current/teen_talaq.pdf).
Allah knows Best!
Darul Uloom Deoband