Have u really ordered 3 @ 480 pounds a pop?because two things come to mind a) why not wait for the 6990s or 595s and get two ? and B) can u actually fit three on a motherboard...
A) 3x580 will most likely be faster and not like 2x590 that is more 4x580 aka 575 strangled to death to use around 300W before OC and 2x6990 i asume will be slower than 4x6970 if not due to internal restrictions it will be because of lower clocks and specs to meet the same 300W limit.
So in short i dont see any change from previous genearations where singlecards in pair was faster than its dualGPU sibbling.
Dual GPUcards only real purpos is PR for having the fastest card and for apps that can use all 1024cudacores of the 590 and/or you dont have a MB to support 4 separate GPU:s but willing to pay the price of less perfomance to get 4 GPU:s
Besides 590 will be a very very limited card and have cherry picked GF110 GPU:s for minimal leakage to save power and price thereafter and nvidia expect they wont make any money of that card either, just PR and crash the party for AMD and 6990.
B) You can watercool this cards and with 3 slots/card it still fit a elite group of uber highend X58 that has 7 PCIe-slots from ASUS, EVGA, Gigabyte etc.
One of the upcomming EK block might fit this card as it shares PCB with what they make a block for and only two memorychip differ those two cards.
Ths way a 3slot card becomes a 2slot card and fit many, many more MB.
C) 590 will not have 6GB framebuffer to match this cards 3GB and 1.5GB+ is what you use in 2560x1600+ with eyecandy even if surround is the targetgroup for this 3GB card. 590 will only have 3GB framebufer and that is 1,5GB per GPU and usable.