The link raina gave points to s3m module file format, which contains the ST3 samplefileformat. According to FILExt
, ST3 could also indicate a Star 3 Midi Karaoke file.
Star3 is a proprietary format created by Creative Labs Korea and is a compressed midi file with lyrics and sometimes an image. Playable with Star3 Karaoke and Real Orchestra players. [link
Google does give some hits, like this MIDI editor
or some (dead) links to player downloads
, but no further description.