Very hard to answer without knowing resolution....makes difference if 1920 x 1080 on 1 screen versus 5760 x 1080 on 3 screens .... and depending on ya card can be $450 ...
What is worth it to one person may not be worth it to another. So you decide....... here's the numbers:
http://us.hardware.info/reviews/4632/35/geforce-gtx-700-series-sli-review-geforce-gtx-760770780-in-sli-and-3-way-sli-geforce-gtx-780---scaling
Metro 2033 @ very high shows a 34,6% improvement from 2 to 3 cards ....more so at 5760.
Elder Scrolls @ high gets just 3.8%, on 1920 .... but 38% at 5760
BF3 @ Ultra goes up 62,6% at 1920 but only 22,6% at 5760
Dirt Showdown @ Ultra gets 42,6% at 1920 but only 29,2% at 5760.
Crysis 3 gets just 7$ at 1920 but 50,9% at 5760
SLI and especially 5760 performance tends to improve much with driver improvements so given that the article is a year old, you will see much better scaling today.