'in' is usually used when something is contained within something else, like "The fish is swimming in the water." 'on' is when something is put above something else (if that makes sense), like "The plates go on the table." The plates are just on the surface of the table, not inside of the table. 'at' is used to describe location, like "I'll be at the park" but can also be used for time, like "I'll be there at 12:00." Entiende mi explicación? Yo sabe un poco de español, entonces puedo tratar a Usted.