I think at this point, there's probably no need for a physical attachment to let you do this. Everything is wireless now.

Also, I don't know how the 3DS' 3D would work on a TV.. and multiple screens would look sort of odd on a TV, too. It'll be cool if there's some sort of compatability between the Wii U and DS/3DS, but I think with the 3DS being what it is, there's no real need to view it on a TV rather than the dual screens.