So... is Nana Maru San Batsu finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: September 20, 2017
Manga finished: October 02, 2020
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Nana Maru San Batsu follows Shiki Koshiyama, a newly enrolled high school student.
He's unexpectedly drawn into the world of competitive quiz games by the Quiz Study Group and its charismatic leader, a senior student.
Alongside his classmate Mari, Shiki explores this fast-paced and intellectually stimulating arena, facing challenges and discoveries along the way.
The anime explores how Shiki adapts to this new environment and the impact competitive quizzing has on his high school life.