For a three monitor setup you're better off with a 7950 or 7970 rather than a GTX660ti or 670. The AMD cards are both 3GB and have more memory bandwidth - 384 bit memory interface on both AMD's, 192 bit on the 660ti, and 256 bit on the 670. To put those numbers into perspective, the 660ti has somewhere around 140GB/sec in actual memory bandwidth. The 7950 is about 100GB/sec faster, and the 7970 is ~30GB/sec faster still. They'll also overclock better than the NVidia cards if that's a concern at all.
I think the only time you could justify buying an NVidia card is if you were gaming on a single high-res monitor, and even then it's somewhat dodgy since NVidia slapped down the vendors for overvolting. Even before that, I thought the 670 was the only NVidia card worth looking at, simply because you could o/c it to perform like a 680 - now you can't. AMD cards are (imo) a better value across the board.