I am planning to visit Zhangjiajie on 8 to 11 september 2012.

Hi, actually Wulingyuan Scenic Area includes Zhangjiajie Scenic Area, Tianzi Mountain and Tianmenshan cause these attractions are under the management of Wulingyuan. Since you wanna go to Wulingyuan, Zhangjiajie and Tianmenshan, here is my suggestion about your itinerary.
Wulingyuan Scenic Area - Zhangjiajie Scenic Spot - Tianmenshan and the skywalk
At Zhangjiajie Central Bus Station, there is direct bus for Wulingyuan Scenic Area. After visiting Wulingyuan, you can take a direct bus there to Zhangjiajie Scenic Area. And Tianmenshan is only about 5 km from Zhangjiajie Village, so it's easy to get there. And the skywalk at Tianmenshan would give you a distinctive experience.
By the way, three days is certainly enough for your traveling.