There are no "Disc 2" or "Disc 3" for Blue Dragon on Xbox 360. It is a single-disc title (DVD-9).
If you have multiple discs labeled "Blue Dragon 2" or "3," they are likely:
Game Info: Developed by Mistwalker, art by Akira Toriyama (Dragon Ball). Released 2006 (JP) / 2007 (WW). Available digitally on Xbox 360 Marketplace and backward compatible on Xbox One/Series X|S.