I get far far more than 3 hours. I'm not sure why you're mentioning standby mode as that's not even relevant. I'm talking 5 hours of nonstop gaming and still not having a low battery. With card in.What? Mine was the launch DSTWO and made long before they even had the chance to knock them off.
Wait, if your system only gets 3 hours, that is a battery drain. My old3DS has been in standby for several days now and still has battery life.
Edit: on N3DS







