Obviously, some mobos need it to recognize that a slot is occupied (more info here: http://blog.zorinaq.com/?e=42)
By the way, I've got the same PSU as yourself. Are you sure that it is adequate for three R9 280x?
Cheers
I don't believe that the AX860 is a good choice for 3 x R9 280X, measured 1 x R9 280X (undervolted) with a current clamb, at full hashing the power was about 270W, that will be 810W in total, leaving 50W for everything else, you can properly squeeze 5-10% more of of the AX, but I would not recommend it, as the derating of the components will proberly lead to short life time of the PSU. I believe that the AX860 is a PSU you can have for many many years if you dont punish it too hard.