三(sān)年(nián)前(qián) means three years before the speech time, which is normally translated as ‘three years ago’, and is definitely represent the past tense.
前(qián)三(sān)年(nián) means the first three years of a period of time, which can be used in either the past tense or the future tense. For example:
Tā sānnián qián qù guò Běijīng
She three years ago go aspect marker Beijing
She had been to Beijing three years ago.
Tā zhǔnbèi qián sānnián xué yīngyǔ
She prepare first three years learn English
She prepared to learn English in the first three years.