Is Temple Of Heaven near Tiananmen Square and Forbidden City? Can we visit these 3 spots in 1 day?

Yes, the distance is not far and you can visit them in one day.
In the morning, have a stroll on Tiananmen Square first, then walk north to reach Forbidden City. Then in the afternoon, continue to visit the Temple of Heaven. Have fun!

Thanks elena for your reply! Can you recommend a nice affordable place to have lunch before touring temple of heaven?

For JSU. I hope this still helps you. Although I have no idea of your preferring flavor, I happen to know a resto named Da Wan Ju, which offers great Chinese cuisine with reasonable price. It is located at no. 55-1, Tiantan Road, Dongcheng District (close to the north gate of the attraction).