Chongqing to 3 gorges dam. Can this be done in 1 day by public Transport(train /bus)

Whichever way you choose, the tour to the dam cannot be finished in a day by public transportation:
If by train, you can go to Yichang first. After arriving, take bus no.4 to Yemingzhu Station. Then take tourist bus no. 8 that can take you to 3 gorges directly. In this way, one and a half days are recommended.

Or you can take a cruise at Chongqing Chaotianmen Port to have the Three Gorges tour, which includes the visiting of the dam. It requires three days or so.