"Sannen Kimengumi" is a comedic manga series set in a Japanese middle school, focusing on the misadventures of a group of misfit students in Class 3-3. The central characters—Shinobu Ijuin, Hajime Kotosaka, and Kazuma Takigawa—form the core trio, often referred to as the "Three-Year Trio." Each character embodies distinct personality traits that drive the humor and chaos of the series. Shinobu is the de facto leader, known for his over-the-top schemes and relentless energy. Hajime, the more level-headed of the group, often serves as the voice of reason, though he is not immune to getting swept up in their antics. Kazuma, the most eccentric, frequently contributes absurd ideas and unpredictable behavior.

The narrative revolves around the trio's attempts to navigate school life, often clashing with their strict teacher, Mr. Dekapan, who is determined to instill discipline in his unruly students. The dynamic between the students and their teacher is a recurring source of conflict and comedy, with the trio devising elaborate plans to outwit Dekapan, only for their schemes to backfire hilariously. The series also features a colorful supporting cast, including classmates and school staff, who add to the chaotic atmosphere.

Episodic in nature, the manga explores a variety of scenarios, from school festivals and exams to bizarre competitions and personal rivalries. The humor is rooted in slapstick, absurdity, and exaggerated character interactions, often pushing the boundaries of logic and reality for comedic effect. Despite the frequent chaos, the series occasionally touches on themes of friendship, perseverance, and the challenges of adolescence, though these moments are typically overshadowed by the relentless comedic tone.

The setting remains largely confined to the school environment, with occasional excursions into the surrounding neighborhood. The school itself becomes a character of sorts, serving as the backdrop for the trio's endless antics and the source of many recurring gags. The series maintains a lighthearted, fast-paced rhythm, with each chapter delivering self-contained stories that contribute to the overall comedic tapestry of the manga.