what's the weather like in March? I'm travelling to beijing, chengdu and shanghai

March is the start of Spring for most places in China, but it is still a little bit cold in north part of China. So I suggest you to take thin sweaters, thick coats and T-shirts.
T-shirts with coats can be used in Shanghai and Chengdu. The sweaters and coat can be used in Beijing.
Hope this piece of information is helpful.