Flour Blend for Seoul Chicken?
Honestly, all three of those will crisp up very quickly, and just about any ratio will produce a crispy end product. The blend may be for textural reasons; potato starch is typically very fine and produces a tempura-like shell, while rice flour can have little bits of individual rice grains which produces a "chunkier" texture.

I probably wouldn't try to use pure starch as it will brown and burn very easily, but you probably don't even need all three ingredients. Personally, I'd start with a 50/50 blend of potato starch and rice flour and adjust from there. You could use tapioca starch instead of potato if it's easier for you to find locally, but definitely keep rice flour in the mix.