I was adopted thru the 3rd process petition thru immediate relative I was under age not married, want to know if thru that I can fill in an n-600

There is not enough information to assess the matter. You may be eligible to file a Form N-600. I recommend you take your adoption records to an experienced immigration attorney for review. You can read more about deriving citizen up through your parents at http://myattorneyusa.com/citizenship-and-naturalization.