Here is the full list of all 26 The Legend of Heroes: Trails in the Sky the 3rd achievements.
Finish the game on Easy or above.
Finish the game on Normal or above.
Finish the game on Hard or above.
Finish the game on Nightmare.
Complete Chapter 8.
Obtain a special quartz from cowardly deeds.
Complete all Star Doors.
Complete all Sun Doors.
Defeat all Abyss fiends.
Read all of the books in the Garden.
Obtain a special quartz from brave deeds.
Complete Chapter 7.
Complete Chapter 6.
Complete the Farewall Arena on Nightmare.
Complete "Who Wants to Be a Mirannaire?" Maniac mode.
Complete all minigames on any difficulty.
Carefully examine nearly every chest that you come across before the final chapter.
Beat the ultimate opponent on the highest difficulty.
Complete all Moon Doors.
Read a book you wish you hadn't...
Complete Chapter 5.
Complete Chapter 4.
Complete Chapter 3.
Complete Chapter 2.
Complete Chapter 1.
Complete the Prologue.