i got 580SLI, and im consider to add a third card ( got it cheap).
my cpu's are e5620 @ 3.6 GHZ, and i got a lot of HDD's (9-10)
if i'll take the cables that connected to the optional 6PIN for each PSU, i will be able to connect the third card.
 
the PSU will handle all that hardware? should i consider add a second PSU? (got  extra HX1000).
 
3.6 GHZ for the CPU's is enough  for 3WAY? i have to squeeze them more to have "juice"?
 
why 3way? i'm running on 3 Dell 2311 (5760X1080),
 
thanks.

For your choice in CPU, you will be fine.
 
But, to run three-way, you'll need an adapter to plug in the additional PCIe power connector (near the back-panel headers) to run the lower 4 slots. Something like this:
 
http://www.newegg.com/Product/Product.aspx?Item=N82E16812200106
 
will work fine.
 
 

i already connected this power connector.
 

if you oc the cards you may be getting a little tight on the power supply at 1200w. way to know for sure is use a kill-a-watt to measure your actual draw from wall. if you have 10 hdd and lots of fans, you are pushing it i with an oc on the cards and cpu's. cant hurt to add the second psu. I would run the 1200 for the 3 cards/fans and the 1000 for the mb/hard drives. right now with 3 560 classys I run 2 corsair 850's 1 for the 3 cards and the other for the mb. I have a third 350watt psu for pumps and fans in my external rad system. will be upgrading to that new evga 1500 watt if they ever release it.

My 3 580's easily tax the AX1200 that supplys them.. really heats up when the cards ramp up.  the other AX1200 runs everything else including 10 drives, pumps and fans and a pair of overclocked X5690's.. stays nice and cool.