1, 2 or 3 GPUs

Solution
Three GPU's currently do not add much more performance over two (http://www.tomshardware.com/reviews/geforce-gtx-760-vs-780-sli,3686-6.html) so I would just buy two.

The gtx 780 ti and gtx titan are pretty close in performance (http://gpuboss.com/gpus/GeForce-GTX-TITAN-vs-GeForce-GTX-780-Ti), The GTX 780ti is a bit better in most benchmarks.
You would need a VERY VERY POWERFUL Power Supply if you want to go with 3 or even 2 GPUs of those.

The GTX 780 Ti is better than GTX Titan, however, if you want to get a GTX Titan BLACK, then that would win them both.
 
A better question might be what are you planning to do with them? I have the original gtx titan and can run 3x 3d asus monitors with high settings with no problems. Multimonitor setups i'd go with 6gb and single monitor stick with the 3gb. Just my 2 cents.
 
Three GPU's currently do not add much more performance over two (http://www.tomshardware.com/reviews/geforce-gtx-760-vs-780-sli,3686-6.html) so I would just buy two.

The gtx 780 ti and gtx titan are pretty close in performance (http://gpuboss.com/gpus/GeForce-GTX-TITAN-vs-GeForce-GTX-780-Ti), The GTX 780ti is a bit better in most benchmarks.
 
Solution
SLI doesn't tend to like 3 cards - not entirely sure why, but it's probably the protocall itself. Regardless, you're in the realm of serious overkill as it is. Unless you're wanting to play the latest games across 3 monitors in 4k resolution on Ultra settings, there's little point in getting even two 780 ti's (which is better than the Titan, no contest) 😵. For the sake of future proofing, I would advise going for one, at max two, and replacing them when they age out.