Yes all skylake will support both DDR3 and DDR4. However, it's the motherboard that get's the say so on which one it will use (or both).
Keep in mind the DDR3 it supportsIS NOT STANDARD DDR3, but rather the DDR3L (low voltage) version. Which is rare to find.
DDR4 is the same price as DDR3, unless your somewhere else besides the UK or US.