John Shephard in The Tarot Trumps: The Cosmos in Miniature (1985) defines the three levels of seven cards as representing the Realms of Man, of the Soul and of Eternity.
In The Tarot of the Bohemians Papus describes the major arcana as representing the septenary of planets acting upon the three worlds.
De Guaita wrote two volumes of a proposed three volume set on occultism, with a structure based upon a three septenaries division of the tarot trumps-
Wirth (Tarot of the Magicians) also discusses the three septenaries of the tarot -
Eilteen Connolly in Tarot A New Handbook for the Apprentice (1977) also writes of The Septenaries: The Major Arcana are divided into three groups of seven cards each, called SEPTENARIES.
I haven't been able to check but if memory serves me right Sadhu speaks of both three septenaries and seven ternaries?
Hilton Hotema (The Land of Light: The Tarot, 1996), quoting Eliphas Levi on the Isiaac Tablet (and its relationship to the tarot) wrote:
The three septenaries furnish the absolute number of the three worlds, the aerial, fluidal and physical, as well as the complete number of primitive letters, to which a complementary sign was added, like zero to the nine numerals
Frederick Graves (Window of the Tarot, 1973) wrote that the trumps are: "are classified into three septenaries and the zero card, The Fool."
Papus allocates the first Septenary to Man, the second to God, the third to the Universe