They compete for consumer mindshare, time, money & pocket space, so yes, they do compete with Apple.
You could say that about any consumer product. If some decides to buy a book published by RandomHouse instead of a Nintendo game, are Nintendo competing with them too? Nobody buys a iOS for the games. They may buy some games when they have one, but they don't buy one as a gaming device.
If 3DS ends up with a Kindle-like App (which is likely at some point), then NO, even then Random House Publishing and Nintendo are not competing because Nintendo doesn't sell books.
iDevice & 3DS are portable consumer multi-tasking multi-media/game devices so they overlap in market appeal and functionality.
If the next best comparison you have is a Toaster vs 3DS then please, save your breath.