Assalaamu alaykum. There is a quote by Ibn Taymiyah that circulates on internet and claims that he said that Imam Ahmad changed his verdicts on the triple divorce at a time: Ibn Taymiyah said:
“Imam Ahmed used to consider combining three divorces permissible, then he retracted from that and said, '(When) I contemplated over the Quran, I found the Talaaq (divorce) to be Talaaq Ar Raj'ee (where a man can live with his wife again without a new marriage), or he said something like this, and he established his Madhhab (School of Thought) on that (i.e the last opinion), and the majority of his companions are on that (too), and it becomes clear from the hadith of Fatimah that she was divorced three times differently, and not unitedly, and two hadiths are authentic according to him, the one who combines three divorces, then (three divorces) have not become obligatory upon him except only one, and nothing is proven from the Prophet, sallallaahu ʻalayhi wa sallam, against this; rather, the Quran is in agreement with this.” [Fataawa Ibn Taymiyyah (8/423]
Is it true that Imam Ahmad changed his views? From where did Ibn Taymiyah quote this, and why is that all the Hanbali Fiqh books never mentioned this change of opinion of Imam Ahmad?

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, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

The statement of Ibn Taymiyyah regarding the view of Imaam Ahmad on the triple divorce is related to the takleefi ruling on the action (i.e. whether the action of the servants is permissible or not). In other words, is the triple divorce a permissible Sunni divorce (in conformity with the Sunnah) or a prohibited Bidʻi divorce (innovative divorce, not in conformity with the Sunnah)?

This is different from the wadhi'i ruling on the action (i.e. whether the action of the servants is valid or defective). In other words, does the triple divorce take effect as one divorce or three occurrences of divorce? These are two different issues. The scholarly view that the triple divorce is prohibited does not necessitate that such a divorce counts as one divorce. This is similar to divorce during the woman's menstruation; it is ruled as a Bidʻi divorce (not in conformity with the Sunnah) and prohibited according to the majority of scholars; however, it takes effect according to most of them.

That said, you come to realize that the statement of Ibn Taymiyyah about Imaam Ahmad retracting his view about the permissibility of issuing three divorces at once does not necessarily mean that he held that the triple divorce counts as one divorce; rather, it means that he stated that it is prohibited. There are two views reported from Imaam Ahmad in the Hanbali books in this regard; one declares permissibility of the triple divorce, and the other declares its prohibition. The official view of the Hanbali school is that triple divorce is prohibited, and this view was chosen by many of the Hanbali scholars as cited in their books. For instance, Ibn Qudaamah wrote, “The second view (narrated from Imaam Ahmad) is that issuing three (pronunciations of divorce) in one session is a Bidʻi divorce and accordingly prohibited. This view was adopted by Abu Bakr and Abu Hafs.” [Al-Mughni, 7/368]

Al-Mardaawi wrote:

“If he divorces her thrice in one session while she is in a state of purity during which they have not engaged in intimate relations, then such a divorce is disliked. There are two reported views regarding its prohibition. These two views were cited in Al-Hidaayah, Al-Mustawʻib, Al-Haadi, and Al-Kaafi (books) without declaring any of them preponderant. One view suggests that it is prohibited, which is the official view of the Hanbali school, and it was stated in the narration from Imaam Ahmad on the authority of Ibn Haani', Abu Daawood, Al-Marwathi, Abu Bakr ibn Sadaqah, and Abu Al-Haarith. This view was adopted by the majority of the Hanbali scholars and was declared preponderant in ʻUmdat Al-Ahkaam, Al-Wajeez, Muntakhab Al-Aadami Al-Baghdaadi, and other books...” [Al-Insaaf fi Maʻrifat Ar-Raajih min Al-Khilaaf, 8/451]

The statements of some of the Hanbali scholars suggest that Imaam Ahmad retracted the first view reported on his authority. Ibn Abu Moosa wrote:

“Triple divorce issued in a single sentence for the wife who is in a state of ritual purity during which they have not engaged in intimate relations is considered a Sunni divorce according to one of the two reported views (of Imaam Ahmad). The other view suggests that the recommended Sunni divorce is that he issues one single divorce while the wife is in a state of ritual purity during which they have not had intimate relations and then leaves her to observe her ʻIddah (post-divorce waiting period). If he wishes to revoke the divorce and take her back in marriage, he can do so during the ʻIddah. If he does not wish to revoke the divorce, then she becomes finally divorced and the marriage bond is terminated as the ʻIddah expires. This is the Sunni divorce that conforms to the Sunnah. He (Imaam Ahmad) said: 'I have contemplated the relevant views and could not find any other way but this one...” [Al-Irshaad]

Allaah knows best.