From Sega Retro
X3V is a voice bank format developed by nineko for use in his xm3smps music conversion tool. It can contain up to 255 YM2612 voices formatted according to the SMPS specifications; additionally, it can store a description for each voice. Other than xm3smps, Puto's xm4smps can also read and write X3V voice banks.
The internal structure of an X3V voice bank is simple. The first byte specifies how many voices there are in the bank (0 to 255); after that, the actual voices begin, in the following format:
|25 bytes||0 to 255 bytes|