Using X3 to mixdown live to X3 track (hands on faders)

That all depends on what you call mixing down live...
 
If you have a DAW controller with lots of knobs and faders you can do your thing "live" while Sonar exports in real time, but as most of us don't have big controller consoles with enough channels to do that we just use automation.
 
The other way would be to output all the audio as separate channels to a real mixer and record the output of the mixer back into Sonar, which can be done easily IF you have enough physical outputs for all the audio.  The PreSonus StudioLive series are great for this as they have all the inputs and outputs you need built in on FireWire so you're keeping everything digital at all times with no audio degradation from conversion.
 
But why bother? Sonar can record automation from the cheapest of controllers in real time.  If you only have one fader (say, a fader port) you just do several passes, recording automation where it's needed.  Then quick export as usual.