First you create your UPC single item.
then you create your Case GTIN packoff items. If there is 3 differerent packoffs off one item, then you have the same UPC and a different GTIN hierarchy for the packoffs.
Examples :
00424444444444 single item...
If I am adding an item that will be sold as Random weight and has no upc on the package, Do I set up as just the final case rather than and item then the case?