iam2thecrowe :
I cant help but think you would be better off selling the 980ti's and upgrading to a newer 1070sli or 1080. They will offer much more reliable performance than 3 cards, you can keep your current motherboard and the new cards are much better for dx12.
True that a 1080 SLI build will offer better performance, but with the decreasing prices of 980tis, and the fact that 3 way SLI is even supported in games is a large selling point for enthusiasts looking to get bang for their buck at the moment.
In contrast, you can get 2x 980tis for the price of a single 1080, and you can choose from any model compared to the very limited range actually available in 1080s at the moment, with most selling out within minutes. (Those that aren't FE anyway).
EDIT: Selling the 980tis could also prove to be difficult without undercutting yourself on sales.
You can get an air cooled 980ti new for $350 at the moment, meaning that the market for second hand Hybrids will be quite small considering people that want water cooled high performance rigs will either go for custom loops, or hybrid/waterforce 1080s.
This means that a standard price for a second hand card like OPs would be about $350, not a great deal considering the original purchase price.
Best to wait until upgrading just before the 1080tis release imo and sell the 980tis then, the performance increase isn't significant enough to warrant selling them now and buying 1080s, but the third 980ti can add a surprising amount to rendering etc. which seems to be the reason most people buy x99 CPUs.
Can't say the same for the fourth card though!

Given that the sound card takes up two slots anyway, getting the new motherboard i've listed also gives upgrade potential in terms of Broadwell E CPUs should that tickle your fancy in the future, as well as a PCI-E SSD if you go two way 1080ti SLI.