Will DS games work on Nintendo 3DS?

Yes, DS games work on a 3DS, except the picture in some games may not fill the entire screen. Also, as I understand it, 3DS cards are slightly larger than DS cards, so will not be able to be played on a DS.

Yes, just as Gameboy Advance games work on the original DS, the DS games should work on the 3DS.

yes, ds games can work on nintendo 3ds. And currectly there is only one card named gateway 3ds can work on 3ds game. But it will be available at the end of Jun.

gatway 3ds: http://r4i-3ds.com/nds-r4-3ds/where-to-buy-the-worlds-first-3ds-flash-card-gateway-3ds-136/