Why popes bless with three fingers according to Aramis’s Jesuit adviser in Three Musketeers

Aramis is one of the The Three Musketeers (Bantam Classic) in Alexandre Dumas’s book. Aramis is a Jesuit Novice who is writing his thesis in preparation for his ordination to the priesthood. Aramis’s adviser is a Jesuit, and his adviser suggested to him the following topic in Latin:
“Ultraque manus in benedicendo clericis inferioribus necessaria est.” In English, “Both hands are necessary for priests of the lower orders when they give benediction.” The Jesuit adviser explained to Aramis:

Read more at Monk’s Hobbit.