The Haunted Mansion
Level select
While playing the game, hold Right, then press X(2), B, Y(2), B, X, A.
Invincibility
While playing the game, hold Right, then press B, X(3), B, X, Y, A.
Weapon upgrade
While playing the game, hold Right, then press B(2), Y(2), X(3), A.
The image featured at the top of this post is ©The Legend of Heroes: Trails of Cold Steel III key art / Original