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.