i heard it will took you 3 acts in nightmare to level up...
What is this NM full clear of which you speak, cousin?
NM is all about picking up the quest reward points and getting to Baal as quickly as possible. Anything more than that is for completists. Hell Pits will provide all the XP one needs before moving on to Eldritch, Pindy, and Baal.
But that's just me. YMMV.