Orthodox source states:“In 1569, the Frankish Pope abolished the use of three fingers in the West, and ordered the sign of the cross to be made with five fingers, and from right to left, instead of left to right. However, this practice was not introduced in the East, neither among the Orthodox, nor among the Monophysites and Nestorians.”
orthodoxwiki.org/Sign_of_the_Cross
If so, that would be Pope
St. Pius V (1566-72) and the time is after the Council of Trent. However it does not seem to have been
abolished, so it is not clear what the Orthodox source means, since use was retained by some as we read in
In A History Of The Mass And Its Ceremonies In The Eastern And Western Church by Rev John O’Brien A.M.When all the ancient practices died away, and the present discipline was introduced, for quite a long time it was the rule to trace the right hand from the right to the left shoulder after having touched the breast, instead of, as now, from the left to the right. The latter came into general use in the time of Pope Pius V. (sixteenth century).
…
Regarding the disposition of the fingers in making this sacred sign, different practices existed, too, at one time. The most general way, however, in the Latin Church was to close the small and annular fingers of the right hand and extend the other three; then to make with the hand thus disposed the required sign. Bishops and the members of the Carthusian and Dominican orders have retained this custom. The two fingers united in this way symbolize the duality of natures in our Divine Lord, against the Eutychians, who maintained that there was but one; and the three other fingers typify the Blessed Trinity (Romsee, iv. 56; Bona, De Divina Psalmodia, p. 507). It will interest the reader to know that our Holy Father the Pope always observes this ancient disposition of the fingers whenever he imparts his blessing, as may be seen from any correct picture representing him in this attitude.
ecatholic2000.com/history7/untitled-122.shtml