Can you buy games using play coins on a 3ds

Yes. If you complete all six AR games, you will be able to go to the AR Games Shop. You can spend them on new levels and features. They can cost 1 to a few coins each.
You can unlock content in 3ds games using the coins (but not all I think). Super Street Fighter IV: 3D Edition lets you trade play coins to eventually get statues.
StreetPass Mii Plaza has mini-games and you can purchase unlocks for a couple of coins.