In small amounts used for thickening foods such as puddings or gravies, you can substitute 1:1 any of these flours/starches: potato, corn, rice, arrowroot. When you get into quantities by the cupful, use one or two level tablespoons less cornstarch per cup of potato flour, depending on the wetness of whatever it is you're baking.
In small amounts (2-3 tsp), they're completely interchangeable. They do have slightly different effects on baked goods, though, so in larger quantities you'll see a difference. I like tapioca starch the best because, when baked with eggs in the mixture, it swells up and kind of poofs. (See the bread mixes from Chebe for an example of this: http://www.chebe.com/) Potato starch has a similar quality to tapioca, but I've found the reaction to be not as strong. It also tends to turn out a bit more gluey in texture. Corn starch doesn't really have these properties, but will help hold your gluten-free baking together. I usually don't use corn starch at all because tapioca and potato work better for what I need them to do.
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