multiple entry visa?

I will be traveling to China beginning of June. I would be flying into shanghai as that is where my friend lives, we plan on visiting Xi'an, Beijing, Shenzhen, and Hong Kong, these are not confirmed plans so I do't have hotel or itinerary for these places. will my visa get denied because I don't have this info. should I obtain a multiple entry visa? secondly, if we do venture into Hong Kong once i return mainland, I have 3 days to leave China, is this correct?