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".

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