You could visit a series of famous attractions in Beijing. One day to explore Tiananmen Square->the Forbidden City-->Beihai Park-->Houhai Bar street. You just have to take a bus or metro to reach Tiananmen Square firstly and then walk to the others one by one from south to north.

On the second day, you could take a metro or bus to Deshengmen bus terminus and then transfer to the bus 919 to Badaling Great Wall, must of Beijing. In the late afternoon, you could go to Xidan commercial street or explore more Beijing hutongs and courtyard.

On the third day, please take the metro Line 5 to visit the Temple of Heaven and then go to Qianmen Street by bus or taxi.

As for details about these places, please read this page firstly: https://www.travelchinaguide.com/cityguides/beijing.htm