The 3DS is a handheld in the DS series, and because of that, it looks a lot like a DS lite or DSI. In Chibi-Robo photo finder, it isn't the easiest to discover by your self. In order to take the photo, you have to take ether a 3DS, DSI or DS lite, put it on a surface so the top screen is facing up, and take a photo of it at an angle. If you want more info, look at its wiki page here.