in your case, you don't need to use Set Analysis at all. Your chart has 3 Dimensions - Channel, Zone, and Product, and you need to calculate total by Zone and Product, disregarding Channel. The syntax is simple, like this:
sum( TOTAL <Zone, Product> Quantity)
If you'd like to learn more about Set Analysis, AGGR, TOTAL, and other advanced techniques, I'm inviting you to listen to my lecture at the Masters Summit for Qlik, or to read my book QlikView Your Business.