Having recently finished (again) The Lord of the Rings trilogy, I wondered what is the earliest known book / novel trilogy in literature? (Yes, I know J.R.R. Tolkien wrote LOTR not as a trilogy but as the first volume of a planned two-volume work, including LOTR and The Silmarillion but the two volume work was never completed and LOTR now stands on its own in what we refer to as a trilogy.)
According to both Wikipedia and Britannica, trilogy is used to describe
a series of three dramas or literary or musical compositions that, although each is in one sense complete, have a close mutual relation and form one theme or develop aspects of one basic concept.
Britannica's trilogy entry goes on further to say (Wiki mentions this as well, but quoting Britannica here):
The term originally referred specifically to a group of three tragedies written by one author for competition. This trilogy constituted the traditional set of plays presented in Athens by a number of competitors at the 5th-century-BC drama festivals known as the Great Dionysia. One of the first authors to present such a trilogy was Aeschylus, whose Oresteia is the only surviving example from that time.
So we have there an example of what is thought to be the earliest drama / play in trilogy form.
The Wikipedia trilogy entry mentions the epic Mahabharata (attributed to Vyāsa). According to Wiki:
The Mahābhārata is the longest epic poem known and has been described as "the longest poem ever written."
with origins in the 8th or 9th century BCE. That might make it the earliest known trilogy of an epic poem. However, I am having difficulty finding a reference to the earliest known trilogy of books/novels.
Any literature students / professors / historians / enthusiasts (or librarians) out there who may know which trio of books / novels is the earliest known example in literature (and the author)?
EDIT: for the purpose of this question let’s use the Wiki/Britannica definition of trilogy quoted above.