If your setup could benefit from it, then go for it!
Just remember the scaling is probably going to take a good chunk of the third cards performance, and 3GB VRAM is going to limit you quite a bit compared to the performance of tri-sli 780 (compares to if someone had a 1GB version).
But if you're stuck at a low FPS, or only high settings because of screen setup, then why not get another while they're cheap.