How to Get Play Coins on Nintendo 3DS? They Don't Exist – 3DS Uses eShop Funds & Step Tracking for Game Bonuses

Play Coins do not exist on 3DS. They were a DS/Wii feature (earned by walking with system in sleep mode).

3DS Currency: Nintendo eShop Funds

• Add via credit/debit card, PayPal, or Nintendo eShop Card (retail).
• Used for games, DLC, themes, demos.

3DS Step Tracking

• Activity Log tracks steps (via accelerometer) for Game Notes and some game bonuses (e.g., Pokémon X/Y O-Power, Animal Crossing MEOW coupons).
• No universal currency earned.

If you meant StreetPass or Step-based unlocks, those are per-game, not a system currency.