How do you get dragon force on gh3

You have to complete the Career on any difficulty, then beat Lou (the last guitar battle). After that, you will play Through the Fire and the Flames with no rock meter, points gauge or star power and you cannot fail (not you must succeed, but the song won't let you fail). When you have done all that, the song will appear in the Bonus songs section (press the blue key on the setlist). Glad to help! Rock on! (Good luck trying it on Expert!)