ScreamTracker, ModPlug Player, Impulse Tracker and possibly Winamp will open ST3 files.

Try winamp first, if that doesn't work do a google search for ModPlug Player. ScreamTracker is the program that ST3 files are made in and ImpulseTracker can create either .IT or .ST3 files. Screamtracker only runs under DOS, and I think ImpulseTracker does too.