One Nintendo Network ID can be registered to each Nintendo 3DS system. If you want to use your Nintendo Network ID on another Nintendo 3DS system, it's necessary to perform a system transfer.
If you have connected your Wii U to the internet, we strongly recommend you register the same Nintendo Network ID on Nintendo 3DS. Using the same Nintendo Network ID on both systems will combine their Nintendo eShop funds into a single combined balance.
After registering a Nintendo Network ID, you will be unable to change countries and use multiple currencies – for instance, pounds and euros – in Nintendo eShop. The funds in the currency that corresponds to the country setting of your Nintendo Network ID will remain, and any funds in other currencies will be deleted and no longer usable. Please make sure to use up funds in any other currencies before registering a Nintendo Network ID to your Nintendo 3DS system.