What are the SL 3's AUX channels for?
All kinds of things - many undreamed of yet. The Aux input allows you to use three decks with Scratch Live 2.0, record your set (e.g., from a mixer's aux output) or feed a stereo signal to Scratch Live's LiveFeed feature. The Aux output can also be used as a third virtual deck that can be fed from Scratch Live's SP-6 Sample Player. More features for this extra channel will be added over time!
Keep an eye on the scratchlive.net forum - There's bound to be great forum threads on other useful Aux channel tips and tricks!