According to the World Atlas, the population of Tokyo – Yokohama is about 37.8 million. http://www.worldatlas.com/citypops.htm
However, it is said that during the day there are as many as 40 million people in Tokyo.
Where do the 3 million people come from?!
They commute from the other surrounding prefectures like Kanagawa, Saitama, Chiba, etc.