I've just completed full SCS.3m support for Mixxx 1.7.x. It features control of all on-screen functions, complete with center-reset for the sliders that people love so much on the SCS.3d, and even includes "needle drop" functionality where you can use the cross-fader strip to quickly search through a track from beginning to end.
The mapping files and download instructions are here: http://mixxx.org/wiki/doku.php/stanton_scs.3m
There's also a user guide that explains how everything is mapped: http://mixxx.org/wiki/doku.php/stanton_ ... user_guide
Enjoy and let me know what you think!
Also note that Mixxx 1.8 which is due out in the next couple months supports multiple MIDI devices (among lots of other things) so the complete SC System 3 will finally be usable in Mixxx (and you'll be able to mix and match controllers at will. No messing around with special presets!)