There is only one file type that uses the .TRIM.3DS extension:
Trimmed 3DS ROM
A TRIM.3DS file is a trimmed .3DS ROM file created by GodMode9, a file browser for the Nintendo 3DS console. It contains a copy of the data from an original 3DS game cartridge but with all unnecessary filler information removed.
GodMode9 can be used to open TRIM.3DS files on a Nintendo 3DS console. In order to open it on a computer you can remove the ".trim" extension and leave the ".3ds" extension in order to open the file as a 3DS file with 3DS emulators, such as TronDS and Citra.
|Rename ".trim.3ds" to ".3ds" and open as a 3DS file|
Updated: October 19, 2017