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.