Look at the motherboard. Look closely. Notice that there are three really long slots? One is blue, one is white, and one is black. The blue slot has up to 16x normal PCIe bandwidth, but only 8x when another card is in the white slot. The card in the white slot gets 8x normal PCIe bandwidth (when another card is in the blue slot). The card in the black slot only gets 4x normal PCIe bandwidth.
All three cards should work just fine. In general, it would be good to avoid connecting monitors that will show video or other frequent, high-speed graphics changes to that third card in the black slot; though this is by no means a requirement.
Thanks. That makes sense now.