Three days (sān tiān) is a compound noun formed by the numeral 三 (sān, 'three') and the noun 天 (tiān, 'day'). Together, they literally mean 'three days' and function as a time duration phrase. It’s commonly used to express short-term periods in daily life—such as deadlines, recovery times, travel plans, or temporary conditions.

This phrase behaves like a measure word–noun unit: it can follow verbs like 过 (guò, 'to pass'), 需要 (xūyào, 'to need'), or 没有 (méiyǒu, 'not have'), and often appears with time prepositions like 在 (zài, 'in/within') or 从…起 (cóng…qǐ, 'starting from…'). Unlike English, Chinese doesn’t use articles or plural markers here—sān tiān stands alone as a complete temporal expression, and context determines whether it refers to consecutive days, a span, or an approximate period.