noun
riding triple; three people on one bicycle/motorcycle
Slang term for three people riding on a vehicle designed for one or two, especially bicycles and motorcycles. Derived from ケツ (buttocks) and the number three, implying three rear ends on one seat. Often used in contexts of rule-breaking or reckless behavior.
See also: ニケツ
中学生が３ケツで自転車に乗って警察に注意された。
The junior high school students were warned by the police for riding triple on a bicycle.
バイクの３ケツは危ないから絶対にやめよう。
Riding triple on a motorcycle is dangerous, so let's definitely not do it.
ニケツ means riding double (two people on a single-rider vehicle), while ３ケツ specifically means three people.
Slang compound of 三 (three) and ケツ (buttocks, from 尻). The term humorously refers to three people's buttocks on one seat.