Nobody knows yet which socket (1155 or 2011) will be used for cpus past Ivy Bridge. So the best you can do is make a decision on what fits your needs now. I know that's not what you want to hear, but it's the truth.
Now, the only motherboards that support 3 way sli over gen 3 PCI are socket 2011 boards, so that locks you in right there.
With that in mind, a good 3 way sli with Gen 3 support is:
GIGABYTE GA-X79-UD5 LGA 2011 Intel X79 SATA 6Gb/s USB 3.0 Extended ATX Intel Motherboardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16813128531
Notice I went straight past the asrock boards - I just don't trust their quality for an investment of this size. You're paying a hefty price premium for socket 2011, might as well go straight to the good stuff.