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。 ———————— maybe it's little bit complicated ,but if you practice more ,I think you will get it ~

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.