People gather or form groups in small numbers, typically three to five.
几个人、几个人在一起。
明·余继登《典故纪闻》卷十二三五成群，高谈嬉笑。”
During the break, students gathered in small groups, talking and laughing.
Learn the next connected idiom: 三五成群 (sān wǔ chéng qún) → 群策群力 (qún cè qún lì)
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