I'm almost positive you would see better performance from 2 way sli of 980s for a very similar price. 3 way sli doesn't scale well. Then your psu will be fine plus you will get better performance in applications that don't support sli
Not enough power. Just three of those card will require 435 Watts of power. I would suggest 1000 watts maybe? Its better to be safe. You can think about upgrading your CPU to a i7 4770K or 4790K? Shouldnt be a huge price difference. But thats a nice build, lots of money tho but nice build!!
I'm almost positive you would see better performance from 2 way sli of 980s for a very similar price. 3 way sli doesn't scale well. Then your psu will be fine plus you will get better performance in applications that don't support sli