This is a difficult question because these are prepositions and it is often necessary to learn how to use them by learning examples. The basic meanings of these three are location. ON = on top of ex the book is on the table. IN = inside within ex The information is in the book AT= where ex I am at home What makes things difficult is that they are often used for things that are not location. ON= about My essay is on my travel in South East Asia. IN = makes the next word an adverb In fact, I do not know. AT = time We came at 8 o'clock. Find a good grammar book and copy the examples on flash cards (See my notebook entry on flash cards and how to use them) then memorize. Hope this helps! John

Prepositions are one of the worst parts of any language. There are some situations, where you can use one and only one of them, having other situations it may depend on a context. ;)