No, you can not use DDR3 on a motherboard that only supports DDR4. There are motherboards for Skylake cpus that do support DDR3, but the memory controller was only designed for DDR4 and DDR3L(low voltage ddr3) and Intel has warned using higher voltage DDR3 may reduce the lifespan of the cpu. Given the price of DDR4 I would just upgrade to DDR4 with the new cpu and motherboard.