In general, these prepositions are used with places in the following manner. "On" refers to being on top of or on the surface of a location. "In" refers to being inside of an objects boundaries. (This one is used with coutries and cities: They are in (England, France, China, New York, Prague, Buenos Aires, etc.) "At" is a general location preposition that doesn't emphasize the relation between the two objects. Looking at the following two statements: "He is at the store," only states that he is at the store and does not emphasize any additional relationship between "him" and "the store." He could be inside, outside, on top, in the parking lot of the store, etc. Whereas, "He is in the store," emphasizes the relative location between "him" and "the store." There is no doubt about it, he is inside (not outside, not on top, not in the parking lot). Example: A: I told John to wait for me at the store. Where is he? ("At" used because this statement provides his general location.) B: He told me to tell you that he is in the store. ("In" used because this statement emphasizes that he is inside the sore.) There are some cases, where these rules don't hold, and unless I'm mistaken you will have to learn these as they arise. One example would be if "Person B" from the conversation suddenly remembered additional information about John's location and said the following: B: He also told me that he is buying some bread. So, he is probably on aisle 4. We could probably find a way to rationalize the usage of "on" in the above statement (something along the lines of he's phisically on the floor of aisle 4), but it might be easier to remember that "on" is used in this situation.

At & In : I am at my mom's place in London.. Means I am at a place which is smaller than London. At & In : I am at London in England.. Again London is smaller than England.. It's a helping verb which assists in comparison or differentiation. In: We are in a Hotel having dinner... Again IN is a helping verb to describe usually & mostly in present tenses and also with other tenses On: I am on my way to London.. Means going or in process . On: I am on my to meet you. This word ON is used for describing the process of our work or meets.

Very good question, i'm waiting for the answers too!