Motherboard performance question

[wetjet43]

When it comes to motherboards, will it affect your rigs performance if you use a motherboard that has 3 pci express 3.0 slots, but only one is configured for x16 performance, and the rest drop to x8 if you run 3 GPU's in crossfire mode? Should they all be x16 slots, or does it even matter, as long as you're using pci express 3.0 slots?

[wetjet43]

Does the bandwidth ( x16, x8 ) not matter on a pci-e slot, as long as it's pci-e 3.0?