A third card will add FPS and not like going from one GPU to two. There are exceptions to that statement, like Crysis Warhead @ 2560x. 1920x1200 with high degrees of AA should show some benefits, and 2560x with AA even more so.
I wouldn't bother with Tri-SLi for anything less than 1920x1200 with 8-16x Q AA. A sole exception might be Crysis all VH and that's one game.
If PhysX takes off something like two 285s with a 9600GT for PhysX duty would also be a great option.
Some of you may find the below Anandtech article useful. My big criticism against it would be they limit it to 4xAA use.
8-16xAA @ 1920x-2560x is what needs to be shown for 3 GPUs and up.
The below X-Bit article seems to suggest something like 2 GTX 280s may not be enough for 2560x with 16xAA in some titles-