I think most ST3 users moved on to IT, but I know at least one guy on #MilkyTracker who still uses ST3. Well, that might not be the case anymore since we told him about Schism Tracker.
He does have an old comp though, so he might not have given up ST3 entirely.
ST3 has one attractive, unique quality though which I haven't seen in other publicly released trackers and that's the ability to write both AdLib and sample based modules. I'm thinking of really tapping this at some point and making some hybrid S3Ms which use both samples and FM synthesis. I made some tests already though, and it sounds awesome with a GUS taking care of the samples and an SB handling the FM with the OPL.