You don't have to create duplicate products to get a design to show up in all three categories. Now that Zazzle groups similar items, all you have to do is move one product from a group into each of the categories. I've done that by finding products Zazzle's system is grouping together and splitting those up into different categories.
If you are a shop keeper that organizes their store by design specific categories where each design has it's own category (folder), you might be shaking your head about now thinking you don't want to orphan products in a broad categories in case you never find them again. You can still take advantage of Zazzle's grouping if you create a specific folder for the orphaned products from other areas of your store. For instance inside the Halloween category, create a category for "Patterns" and add just one product from each of the patterns in your store that matchs the Halloween holiday.
These ideas might make more sense to experienced shop keepers. If you're pretty new to the system and need a visual, here is a screen shot of the "edit product details" screen. When you choose edit from a product page, you can move a single product by changing the store category.
Orange with White Polka Dots Case For The Iphone 4 by designs4you
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