Here is how I do it:
HttpContext.Current.Server.MapPath(@"~\~\~\Content\")
If you really need the grandparent path, you can get it from the root path using Path.GetDirectoryName()
:
string root = Server.MapPath("~");
string parent = Path.GetDirectoryName(root);
string grandParent = Path.GetDirectoryName(parent);
But your web app very likely won't have permission to read or write there - I'm not sure what you're going to do with it.