Depends. What 3D display are you using? If you're using a 3DTV like a plasma or LCD that you can get at Best Buy, then you're going to be limited to 720p in 3D at 60 frames per second. Some 3DTV's will actually allow you to do 1080p at 24 frames per second, but who wants to play a game thats limited to only 24 frames? (it's a bandwidth problem inherent in current hdmi chipsets). By late 2012, we could see the first 3DTV's shipping with a new revision of hdmi that has enough bandwidth for 1080p at 60 frames to each eye.
Still, it might actually be early 2013 before we see such 3DTV's at the local Best Buy or Fry's.
Anyways, if you're limited to 720p @60hz 3D, then you'd do fine with a 560 Ti.
If you get one of the 3D computer monitors that are capable of 1080p at 60 frames (via DisplayPort or Dual DVI), then you're going to need more horsepower to get a game like Battlefield 3 running in 3D with a solid framerate. You going to need one of the most powerful cards available.