He would be the III because he is the third in the blood line with the name. If you look at the history of European kings, this often happened.
Your grandfather's father is your great grandfather.
He would be your paternal great grandfather
"Paternal" means "on your father's side". Your paternal great-grandfather would be one of your father's grandfathers.
Your grandfather's father is your great grandfather.
Your father's brother's son is your first cousin. Your father's brother's grandson is your first cousin, once removed.
Your aunt's great uncle's father is her great grandfather, or is not related to her at all if the great uncle married a sister of her grandfather).He would then be your great great grandfather - or not related to you at all.
Your father's grandfather is your great grandfather.
Your grandfather's father is your great grandfather.
I would think that would make him your great grandpa!
Your father's grandfather is your great grandfather. Your great grandfather's daughter could be your father's aunt and your great aunt. The grandson of your great aunt is your second cousin. However, your great grandfather's daughter could also be your father's mother and your grandmother. Then you, one of your brothers, or one of your first cousins, is the grandson in question.
your great grandfather
Your father's grandfather is your great grandfather.