CoffeeLake will use the 300 series chipset, and will not be backwards compatible with the 100 or 200 series.
100 series boards exist(ed) with DDR3L support, I don't believe any 200 series boards supported anything other than DDR4.
While theoretically, CoffeeLake *could* support DDR3L (and likely will in laptops), on a Desktop platform, the only chipsets available are likely to only support DDR4.
TL;DR. Theoretically, yes. In practice (for desktops), they won't.