I researched this a few months ago. Unfortunately, my docuмentation and references are on my old computer which is no longer accessible. I was doing it for my own understanding so I did not worry about keeping that information.
The change from midnight to a three hour fast was made by Pope Pius XII. The change was not made for the benefit of the faithful, but rather for the clergy. In the past, priests really only said Mass on a Sunday in their home parish or where they happened to be on a Sunday. Even if they said three Masses due to the numbers of faithful at the parish, he seldom traveled nor was it thought that most priests would be overly burdened in waiting to break their fast until after he said all of the Masses he was to say on any particular day. With new transportation technology and larger Catholic populations during the early 20th Century, the pope noted that many priests, in order to fulfill the needs of the faithful, were having to say Mass at one parish and travel by train or automobile to another, and sometimes, to a third. The pope came to realize that this was a risk to the health and safety to many priests and, so, he reduced the fast to three hours in order to allow priests, when needed, to take nourishment after a Mass even when he was to say a Mass later that day. The pope clearly spelled out these reasons in the docuмent that established the three hour fast. The change was made to apply universally since it would not be appropriate to make different fasting laws for clergy that did not apply to the faithful.
The docuмent that further changed the Eucharistic fast to one hour under "Pope" Montini appears in the Acts of the Apostolic See. It is not a papal docuмent but, rather, is from one of the Vatican congregations and is unsigned. It is very terse and simply states that the Eucharistic fast has been changed to one hour before the reception of Holy Communion and is made in response to requests made for pastoral reasons.