Very few motherboards for Skylake support regular DDR3; a fair number support DDR3L (1.35V and lower), which almost nobody has. Most are DDR4 only.
This one and a very similar variant, both by Asus, are the only ones I have heard of claiming to support regular DDR3 up to 1.65V:
https://www.asus.com/Motherboards/Z170M-E-D3/
As of right now, DDR4 is not an "upgrade" from DDR3. They have increased the speed but also doubled the latency, with the net effect being that the faster kits of DDR3 and even some mid-range ones will perform as well or better.
I am going through almost exactly the same dilemma - wondering if my old DDR3 set will work with the new i5-6600k and the motherboard mentioned above. And of course mine is 1.7V by default, making it even more complicated. The issue is that anything over 1.35V could allegedly burn up the memory controller according to some people; according to others it's not much to worry about. Not many straight answers out there. I've even written to Asus but haven't heard anything back.
But if you have DDR3 of 1.5V-1.65V, that board mentioned above is likely the one for you.