I think the new AO3 filter functionality is still doing a slow roll-out, so you may or may not have it yet, but if you don't, you should soon.

I do, and they are SO GREAT. Negative tag filters! Much better rating/warnings options! Huge bookmark search improvements! I haven't had much of a chance to play with it yet, but the crossover filtering on its own has already improved my life dramatically (Today when I had a sudden hankering I could actually get all the Casablanca fic that was just about Casablanca without having to jump through hoops!)

So much kudos to everyone involved in this!