3x PCI means it has 3 PCI bus slots
more info here http://en.wikipedia.org/wiki/Conventional_PCI
PCIe (express) is different and is in no way compatible with PCI.
the x16/x4/x1 refers to the amount of lanes the slot has (and max throughput)
the PCIe 3.0 refers to the revision of the PCIe slot. A PCIe 3 video card will work in a PCIe 2 slot.