Where is the Tourist bus terminal in Zhouzhuang Water town?

The bus station is located at No. 395 of Daqiao Road (near Yunting Road). It is about 1.3 miles between the bus station and the entrance, you can reach by walking.
Yes, it is the same bus station that you arrive from Shanghai.

I was told that the only tour bus that goes there is off Exit 3 of Cao Xi Road station?

The Tourist Distributing Center of Shanghai is located near to the Exit 1 of Caoxi Road Station.
There are several direct coaches that can take you to the water town. But sorry that I don't know the exact schedule.
In addition, General Bus Station and South Bus Station also have direct long-distance buses to the destination.