Well, all of the three places are in summer then, Beijing is dry but cool at early morning, so it is suggested that you don't go out at the midnoon. Yangtze and Shanghai should be muggy. There will be occasional rainfall in Shanghai but not a problem. Even in the cloudy day, please protect you from the strong ultraviolet radiation~

Althoug it is hot during July, but you may found less travelers than the other comfortable travel season. Anyway, not a bad idea~