for BF3, a single HD 6970 or GTX 570 will do the Job, even your current GTX 560Ti will run BF3 with 50+ FPS and you'll be more than just fine if you do some overclocks.
if you don't wanna experience the SLI issues and the massive drops in FPS....etc I suggest you go with a single powerful GPU such as the GTX 580 or better wait for the new AMD HD 7K GPUs, said to be released on December 6, the high end HD 7K series are equipped with the new XDR2 rambus memory which is twive faster than GDDR5, besides the 28nm process and 1 GHz GPU frequency.