Extreme guarantee way of making sure they will never find out
Get the digital copy,
Buy it using someone else's wifi you don't know, preferably a business that many people visit a day. If you buy it on your network it will show you downloaded xxx amount of data at that day and time if your friends call your ISP on where you Downloaded data and how much of it.
For good measure hide the SD card where you downloaded it to somewhere nearly impossible to search. Tape it to the ceiling of a closet or put it in an envelope and place it in a drawer.
Lastly turn off your wifi when playing your 3DS game. If you are friends with the friends that you don't want knowing that you got games then its best to not be online at all as they can see what game your playing from the friends list. Disconnect your router if you have to but make sure noone else is using your wireless network. That's just plain rude to disconnect them too.
Hope this helps.
It all starts with this, a jewel containing the ultimate power. -Shadow the Hedgehog