I read a review on the OPs case and it sounds like a magnificent case, expensive (@$800!) but small for radiators.
Without extensive mods which would probably ruin the case, you might want to try and sell it. With the $$ gained from the sale, you could probably swing a CaseLab or Mountain Mod case with tons more room for rads.
You could also do what I did, and buy a Thermaltake Core X9 for less than $200 BUT it won't be nearly as pretty as the In Win S.
Third option would be an external rad such as the MO RA3 or Phobya.
Since the OP appears to have enough $$$ to buy 3 Titan Xs (@$3,000) I would opt for a new and big CaseLab case which would have enough room for any rad combo.
Suffice it to say your case is too small to add enough rad capacity to properly water cool your cpu plus 3 TitanXs.
Here's how I calculate it. At stock your TDP of the cpu = 140Watts and each cpu =250W for a total of 890W. To have a decent watercooling system with low running fans to keep a stock system running quiet even at max you'll need 890w/130 120mm rad capacity or @7 rad spaces. Overclocking the cpu and adjusting the gpus to 275W output will easily bump it @200 watts for so 1090/130 =8.3 rad spaces or at least 2-480 rads.
See where I'm going?
For your setup to have some breathing room to OC I would shoot for a case that would hold 3-480 rads. Mine does but it is frankly too basic for someone who owns what you do case wise. Take time at the CaseLabs or Mountain Mod web site and do it right. You won't regret it.
