D.N.A.: Dark Native Apostle Cheat mode
At the title screen, hold L1 + R1. Keep those buttons held and press Circle(2), Square(2), L2, Circle, L2, Square, Select. Release L1, press Circle, then release R1. Start the game to have all weapons with infinite ammunition.
The image featured at the top of this post is ©The Legend of Heroes: Trails of Cold Steel III key art / Original