Have you tried this?

http://blog.zorinaq.com/images/pcie-short-photo.jpg
http://blog.zorinaq.com/images/pcie-short-schematic.png

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