Did you make sure that you enabled SLI in the Nvidia control panel? I know it's kind of a dumb question to ask, but you'd be surprised at the amount of people I've come across that have dual-GPU setups, and never realized they had to "enable" it 😛
Other than that, make sure you have the latest drivers!!! Newest Nvidia drivers increase Crysis 3 SLI performance by up to 60%!
Other than that, try turning down some performance killers as well. For AA, try sticking on FXAA, Nvidia's AA solution. Decent AA, while using minimal vRAM, and not as hard on performance.
And finally, vRAM. Crysis 3 on ultimate max settings (with 1080p) WILL exceed 2GB of vRAM. I'm going to just assume that you're running the 2GB cards, so yea, you will probably need to turn down that AA, maybe a couple other things. If your exceeding your max vRAM, you can start to get some lag.