Roundtrip Hong Kong to Zhangjiajie

First of all, you are suggested to travel in March. As I know, April is the starting peak season of some attractions, so the visitors will be more and the tickets may be expensive.
Then for your itinerary, I have some suggestions.
Three weeks is such a long time, so you may visit more places.
First, I think you can pay a visit to Shenzhen after HK.
Then you can take a bullet train to Guangzhou.
After that, go directly to ZJJ by train, since I think Changsha is not worth to pay a visit.
Your next destination is Fenghuang, and you can take a bus from ZJJ Central Bus Station there.
At last, you need to take a bus to Changsha and take a flight to HK.
Hv a nice trip!

Thanks very much. We thoughtake of the return by train too. Wouldn"t flying be expensive?

Generally speaking, flight fare is more expensive.
Train is also okay, but you need to take a bullet train to Guangzhou South from Changsha South. Then take metro line 2 and transfer to line 1 at Gongyuanqian, and get off at Guangzhou East, where you can take the train back to HK.

Thanks so much and sorry for the typos etc on the last post. Before coffee. :-) What do you know about Kunming as a tourist area?

Kunming is most famous for the Stone Forest, as I know. Besides, Dianchi Lake, Western Hills and Green Lake Park are also well known. Oh, the show—Dynamic Yunnan is really worth to watch.