其 — the written possessive pronoun

규칙 其 = his / her / its / their in written Chinese: 该公司及其员工。 其原因有三。
其 ALREADY carries possession and takes NO 的: 其原因, never 其的原因.
Set phrases: 其中 (among them), 其他/其余 (the rest), 尤其 (especially), 与其 (rather than).
其中 always looks back at a set already named: 有十个人，其中三个是学生。