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.
Complete Chapter 8.
Complete all Star Doors.
Defeat all Abyss fiends.
Complete Chapter 7.
Complete Chapter 6.
Read all of the books in the Garden.
Complete "Who Wants to Be a Mirannaire?" Maniac mode.
Complete all Sun Doors.
Complete all minigames on any difficulty.
Carefully examine nearly every chest that you come across before the final chapter.
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.
Complete the Farewall Arena on Nightmare.
Obtain a special quartz from brave deeds.
Obtain a special quartz from cowardly deeds.
Finish the game on Hard or above.
Finish the game on Nightmare.
Beat the ultimate opponent on the highest difficulty.