It doesn't matter what slot and how many card are in (unless its only 1) they all are assigned the same gpu number, every time. No matter what, the 280x is always gpu0, and I can confirm this by starting cgminer for gpu0, and physically putting my hand behind it to feel that card getting warm. It's like they have permanent numbering.
I've had card numbers change just from swapping cards around. For instance on one rig the first PCI-e slot is GPU 0, third PCI-e slot is GPU 1. Other rig first PCI-e slot is GPU 1, third PCI-e slot is GPU 0. So you can't assume.