Shanghai subway is convenient, however it can not cover all major attractions, you may also need other modes of transportation to help. To be easy, you should find a travel agency, to be free, you can go by mass transit of the city. If you have enough time, you can explore the city by your own, of course, you need to search more information about this city in advance. I think three days are enough, it want go surrounding places or suburb of Shanghai, you may need more time.