I have studied English for three years "present perfect" means at this time (It is implicit although not indicated by the speaker).
I have been stadying English for three years in this case if you started in for example (january 2013, 2014,and 2015) your are still stadying.
But If you say: I stadied (simple past) English for three years you can't include 2015 So the three years become 2012, 2013 and 2014.
I am sorry It's all I know