I would like to visit Zhangjiajie , my plan is for a 2-3 day trip in National day Holidays.

There is direct train from Dongguan to Zhangjiajie and the train journey takes about 16 hours. Or you may consider taking bullet train from Dongguan to Guangzhou first(around 30 minutes), then take another train to Zhangjiajie. Upon arrival, you can go to Zhangjiajie Central Bus Station to take regular tourist bus to the national forest park. Spend two days exploring the park and stay at a hotel near or in the park. On the third day, you can transfer back to city area and visit Tianmen Mountian, here you will ascend the mountain by cable car, climb the Tianmen Cave and then descend the mountain by sightseeing elevator.

As for the Glass Bridge, you can experience it at Tianmen Mountian or Grand Canyon of Zhangjiajie. By the way, the site will be very crowded during National Holidays, so I suggest you visit it at other time to avoid the holidays if possible.