If the stock pot is only used for stock, or other thin liquids, all that really matters is the bottom. With a reasonably thick bottom of any material there is no need for copper's conductive properties on the sides of the pot. The broth, or other thin liquid, efficiently conducts the heat. Even a pure stainless steel stock pot is viable as heat from hot spots within the pot causes the liquid to flow, mix and equalize. The worse case scenario for the stock is some of the mirepoix might stick. If you are later reducing the stock to a glace de viande, copper is still your best friend.

Please send/post the source for 3mm copper

Thanks