Only these 3 prepositions? Here are some example in sentences: In: I am in the store. On: I am sitting on top of the roof At: I am at the store. In/At in conversation can almost mean the same thing. When you are on something you are not in it or at it. These link can help you learn a little bit more in-depth: http://english.stackexchange.com/questions/87306/difference-between-at-and-in-when-specifying-location More Examples: http://www.english-test.net/forum/ftopic50384.html Hope this helps!

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Someone asked this earlier today. Take a look at this: http://www.italki.com/question/317099