From joedogg, we get a Homebrew which can help you to change dates to get Play Coins of 3DS.
Down this Homebrew which is a 3DS rom from here:
Copy the PlayCoinSetter.3ds to gateway 3ds red card, then select the game PlayCoinSetter.3ds
Now you can click left or right to set the Coins you want to modify , then press A to determine modifications.
Now you will see the count is 300 now!