If you've got rekordbox DJ and have it plugged into the mixer via USB cable, it should detect it and assign the USB on the channel input select switch. Have your two XDJ channels assigned to LINE and use them as normal, but plug the laptop in to the mixer and run music requests on Deck 3 in the software.
I've never used rekordbox DJ nor the 750, so I'm guessing it'll work like that.
Another option is just plugging your phone with Spotify in on the third channel.
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