Hey 熊, I'm the one that posted that comment, I think...
It's pretty simple, I think... especially if you relate it to English. In English we can say
"I studied Chinese for 3 years" or
"I studied 3 years of Chinese"
Chinese grammar is more similar to the second sentence in expressing the length of time that you do certain action. The difference is that the terms "3 years" and "Chinese" are reversed in function, but not in order... let me explain.
In my examples, I wrote:
我学了三年中文了
三年中文 means "3 years of Chinese". More specifically, "3 years" actually modifies the object "Chinese". In your sentence you use 中文三年, which is more similar to the sentence "I studied Chinese for 3 years". However, what you're really saying is "I studied Chinese of 3 years". "Chinese" acts as an adjective and modifies "3 years" which is strange.
Therefore these two sentences are written in the same order (i.e."3 years" and then "Chinese") but in the English version "3 years" is technically the object and "of Chinese" modifies it... English is funny like this, but oh well
"I studied 3 years of Chinese"
我学了三年中文了
Ok, so the second one was
我学中文学了三年了
This format is SUPER useful (especially if the object is really confusing). You can use the verb twice: after the first verb you place the object, and then you place the length of time (or any other modifiers) after the second verb.
For example, If you want to use a complex object like "the game that my friend gave me" you can say
我玩朋友给我的游戏玩得很长时间。
I played the game that my friend gave me for a very long time.
我玩 朋友给我的游戏 玩得 很长时间。
Here are some more examples:
他踢足球踢得很好。 He plays soccer very well.
昨天她跑马拉松跑得很快。 Yesterday she ran a marathon very quickly.
我学马克思主义和毛泽东思想学了四年了。 I studied Marxism and Mao Zedong thought for 4 years.