YOU SAID:
He has been dating her for the last three years
INTO JAPANESE
彼は過去3年間彼女と付き合っています
BACK INTO ENGLISH
He has been with her for the past three years
INTO JAPANESE
彼は過去3年間彼女と一緒にいた
BACK INTO ENGLISH
He has been with her for the past three years
You've done this before, haven't you.