I thought GIGABYTE Z170-HD3 only supports DDR4.
Basically regardless of the brand the overclocking steps would be more or less the same but every CPU is unique, means each chip requires different CPU-related voltages, different strength of integrated memory controller (IMC) resulting in different memory overclock ability and ram-related voltages.

i5 6600K has 3.5GHz base clock and peaks at 3.9GHz with Turbo, generally can be overclocked to 4.3-4.4GHz stable on air cooling (depending on chip of course and cooler).