There is no such place as a "Niedersachen".You must mean "Niedersachsen".But are you aware that "Niedersachsen" is simply German for "Lower Saxony", just like "Deutschland" is German for "Germany"?So you are actually referring to LOWER SAXONY.I somehow get the feeling that you were not aware of this.
It would have been completely impossible for your great-great-great-grandfather to have been born in Lower Saxony, because the German state of Lower Saxony did not yet exist back when he was born.The German state of Lower Saxony was created only 63 short years ago!That means that the source of the information you posted here is incorrect.Where did you get that incorrect information from exactly?Is it the same source where you got that very strange "Hanover, Stadt Hannover" from?
And needless to say, nobody here will be able to assist you anyway if you only give a last name.You're going to have to post a lot more information!
What was your great-great-great-grandfather's full name?When was he born?When did he come to America?Where in the U.S. did he settle?What was his wife's full name?When and where did they get married?What was their religious denomination?
As I said, I am most curious to learn where you got this weird "Hanover, Stadt Hannover, Niedersachsen, Germany" from.I have to strong feeling that it came from somebody who hasn't a clue about Germany!