PCIe x16 slots will accept any x16, x8, x4, x2 or x1 card, so yes, the USB3 card will work.
Just something to bear in mind, some of the older USB3 addon cards use PCIe 1.1, instead of 2.0, and thus will not achieve the full USB3 speed. Further, if the card is PCIe x1 and you try to use two USB3...