Nintendo DS Shop Cards (sometimes called DSi Points Cards) are not compatible with the Nintendo 3DS eShop. If you have an old DS Shop Card sitting around, it won't help you buy 3DS games, DLC, or Virtual Console titles. Here's the breakdown.
For 3DS purchases, you need a Nintendo 3DS eShop Card (also sold as "Nintendo eShop Card" with the 3DS logo). These are widely available at retailers like GameStop, Best Buy, Target, Walmart, and online. They come in denominations of $10, $20, $35, and $50 (USD) and work across the 3DS, Wii U, and Switch eShops since Nintendo unified the account balance system with the Nintendo Account.
If you have an old DS Shop Card, check the expiration date—most expired years ago. Nintendo occasionally offered grace periods, but those windows have long closed.