When do you use 个?
When am I supposed to use 个? For example, you say 三天, but 三个星期。Also 三年, but 三个月。Why is this? Why can't I say 三个年?

Question

When do you use 个?
When am I supposed to use 个? For example, you say 三天, but 三个星期。Also 三年, but 三个月。Why is this?  Why can't I say 三个年?

Answer

first of all ,"个"is a 量词（quantifier），天 星期 年 月，they are 时间名词（Nouns for time）。

But some Nouns for time don't need a quantifier，because from the grammatical function，they are quantifier。we called them 准量词。For example：年、天。They are 准量词，so they don't need the quantifier。

Here are some examples of 准量词：天、年、秒、分钟。 you can't use 个 with them。
but 星期、月、小时，you can use it。
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maybe it's little bit complicated ,but if you practice more ,I think you will get it ~

Answer

三天，三星期 or 三个星期 are both okay. 
三月 and 三个月  have different meanings. 三月=March while 三个月= three months.
We just don't say 三个年, kind of a convention here.