we are going to beijing this june, what is the best option to visit within 3 days 2 night?

The must see places are the Great Wall, the Forbidden City and Tianmen Square.
Besides, you can go to see the Temple of Heaven, Summer Palace and Houhai area.
Houhai area is suggested for you at night.
I think you can refer to https://www.travelchinaguide.com/attraction/beijing/ to find some attractions that you are interested in.