What is difference between 三年前和前三年？

三年前three years ago，前三年the first three years

三年前 three years ago；前三年 the first three years

三年前我开始学汉语。I started learn chinese 3 years ago. Я начал учить китайский 3 года назад.

学汉语的前三年我觉得很难。I felt really hard to learn chinese in the first 3 years. Мне было очень трудно на первые 3 года изучения китайского.

三年前 three years ago.  

三年前我来到菲律宾。I came to the Philippines 3 years ago.

前三年 the first three years 

我出生后的前3年我不会走路。I don’t know how to walk for the first three years after I was born.

 

三年前，three years ago  eg:三年前，我还不会用支付宝，现在几乎每天用支付宝付钱。

前三年，the first three years eg:大学前三年，他们是形影不离的朋友。最后一年，由于各有安排，很少见面。

“三年前”is “three years ago”, while “前三年” is “the first three years”.

we usually use “三年前” to discribe a kind of state that how you are.  for example, “i know nothing about chinese three years ago. ”三年前，我对汉语一无所知。

and “前三年” is just a period of time. for example, i work hard at basics in the first three years, and practice in the last year. 我前三年非常努力地学习基础知识，最后一年实战演练。 

三年前：3 years ago；eg： 我三年前买了这条裙子。I bought this dress three years ago.

前三年：the last 3 years；eg：我们必须要排练，不然就无法像前三年一样三连胜了。 We have to practice or else we won't win like the last three years in a row. 

三年前  three years ago.我三年前还是个学生。  

前三年  for the first three year, It is a time period which is at the beginning of an event  eg: 我在前三年积累了足够的经验和资本。

the first 3 years , it refers to a time period.

3 years ago, refers to a point in time.