An NUS3BANK file is an audio bank used by Super Smash Bros., a Nintendo game for 3DS and Wii U that features popular video game characters. It contains a collection of audio files referenced during gameplay such as songs or character voices.

You can add and remove audio files in NUS3BANK files used by Super Smash Bros. for Wii U with Sm4shExplorer (you need to have the Sm4shMusic plugin installed). This tool allows you to view the whole filesystem of Super Smash Bros., extract files to a folder, open files with a hex editor, and more. The NUS3BANK file typically goes unnoticed by the average user. It is most likely only seen by modders who want to alter gameplay and change the audio.