Well, it depends on what you want to achieve.

If you want to record your sets/mixes, you'd run the tape output on the back of the mixer (also know as a recording output) into the SL3's Aux input (and set the Aux's dipswitch to line level). Or you could also use one of the master or booth outputs as well for recording.

The Aux output on the SL3, you can use as the output of the SP6 sample player, and you'd just run that into one of the line/CD inputs on the mixer. If if you're using three decks, again, run the Aux output into a line/CD input on the mixer.