ancient medieval and modern.. which field is More opportunity and explain them?

I am not getting the question which you are trying to ask. In history all three that is ancient mediaeval and modern History important. If you are talking about competitive examination then modern history is more important because nowadays questions from history are mostly from modern history. And if you are trying to ask about graduation courses then I don't think there is different section like ancient medical on modern history. All comes under history, but even if you want and in this then Ancient History is more important because in ancient history most of architectural and archaeological findings have more importance and scope.