1. Roman History. the office or magistracy of a triumvir.
2. a government of three officers or magistrates functioning jointly.
3. a coalition of three magistrates or rulers for joint administration.
4. any association of three in office or authority.
5. any group or set of three.
The emphasis is on three (3) books that are officially a trilogy, generally featuring the same character(s), but is not restricted that.
We'll see that Brandan Sanderson's Mistborn trilogy has a fourth book, for example (The Alloy of Law), but is set three hundred years after the conclusion of The Hero of Ages and no longer features Vin and Elend as protagonists. We can therefore safely consider the first three books as a proper triumvirate. On the other hand, we know that The Hitchhiker's Guide to the Galaxy is no longer a trilogy in the truest sense, but the Dirk Gently set is. The Annals of the Western Shore by Le Guin consists of three books, each with different characters, settings and story arc, but they are linked by some reoccuring characters and locations. This too is a triumvirate for sharing the same imaginary world.
There are many trilogies. The SF/F/H Series list is a great resource for this challenge.
non est ad astra mollis e terris via sed per lectionem
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