Question: can pcie3 work in pcie 2?
Answer: PCIE 2.0x16 card will work with PCIE 3.0 x 16 slot. But PCIE 2.0x16 card can work only at a max speed of Gen2(5.0GT/s). It can't work at Gen3(PCIE 3.0- 8 GT/s) speed even though the slot(Root complex) support Gen3. The PCIE devices are backward compatible with respect to supported speeds.