Are they the same?
1-önemli değil
2-önemi değil
3-önemi yok

Question

Answer

In my opinion:
1: Most of the times is used as "it`s ok","Its not a problem"," "don`t worry". - Example - "Hallederiz, onemli degil."
2: I never heard of that one, i think it does not exist.
3: Its used as "it doesn't matter". - Example - "Hicbir seyin onemi yok".

Answer

Simply
1-önemli değil: it's ok, no problem
2-önemi değil: doesn't exist
3-önemi yok: it doesn't matter (anymore), not important