This confused me at first, it doesn't actually say where the set of Treasures is to be. If you do mean your play area, I think when you have cards in play face down, they no longer count as 'in play'. There are certain rules questions that arise then, like what happens to the face down set at Clean-up? It's still your turn.
You could turn the set over in the Supply or of course the trash safely, but neither would be as interesting here. You could try turning the set in play sideways or something?
I'm almost sure he meant cards already in play, but I agree that it needs to be specified on the card, like how Necromancer specifies "from the trash".
However, there's no reason that a face-down card would no longer count as in play. A face-down card in the trash still counts as being in the trash (Donald confirmed that you could get a face-down card back with Lurker/Graverobber/Rogue).
I also realize that there's no real reason to use "face down" for this. You could just as easily say "you may play a Copper, a Silver, and a Gold from your hand. If you do, +
. " Or, you could require them to discard those cards from play instead of turning them face down, which would be the same for most situations.
To avoid the beginners question of "do the other treasures still give you their regular money?", you could just make them reveal then discard a Copper, Silver, and a Gold, and give +$12.