I have been looking around the web but have found no answer to this question, it might be a obvious answer but i just want to make sure. Will DDR3 ram work on the new X99 DDR4 motherboards?
Nope. Pins are incompatible and the DIMM won't fit.
No ..... DDR3 dimms have 240 pins and DDR4 dimms have 288 pins
To prevent you from blowing up the sticks and/or board they have keyed it differently so even though the dimms are the same length, they can't fit in the slots.