A preposition is a word that shows where an object is located, or its position in relation to something else. A preposition shows relationship between two or more than two nouns or pronouns.
There are many propositions which are used in English language.
Let us learn at, in and on first.
"AT", "IN" and "ON"
Use 'at', for a specific place or time. Example: We close at 8 o'clock.
Use 'in', for an enclosed or specific space. Example: I live in London.
Use 'on', for a surface, date or day. Example: The cat is sitting on the chair.