Three days in Hanoi
The first two days: Same as One day in Hanoi and Two days in Hanoi
The third day:
You can spend your morning in Bat Trang Ceramic Village. It takes more than an hour to get there by bus number 47 (You can take this bus in Long Bien Bus station). In the village, you can buy some unique souvenirs and try making pottery by yourself.
In the afternoon, you can go back to Long Bien and take a taxi to Vietnam Museum of Ethnology in Cau Giay District. The museum provides a deep and unique insight into the features of different distinct groups in this multi-ethic country through the exhibits. The museum would be super worth visiting if you were fond of anthropology.