'Three years' is a compound numeral-noun phrase formed by the numeral 三 (sān, 'three') followed by the noun 年 (nián, 'year'). Unlike English, Chinese does not use plural markers — 年 remains unchanged regardless of quantity. This phrase functions as a time duration and commonly appears after measure words like 'for' (e.g., 在...里, 已经), or with verbs indicating duration such as 住 (live), 学习 (study), or 工作 (work). It can also serve as a subject or object in sentences.

The phrase is neutral in register and widely used in both spoken and written Chinese — from casual conversation ('我在这里住了三年') to formal contexts ('该项目已运行三年'). It frequently pairs with time-related adverbs like 已经 (already), 还要 (still need to), or 刚刚 (just), and often implies continuity or completion depending on context and aspect particles like 了 or 过.