‘At’ is used when we talk about a specific point or a particular place. For example:

 She arrived at the Mumbai Central station at seven.

 There were very few parents at the meeting.

 We last met at Pragati Maidan.

  

 We use ‘at’ when we talk about buildings like bank, school, supermarket, etc. But we use ‘in’ to emphasise that one is inside the building.

  

  ‘In’ is used when we talk about a larger area or place within the larger space. Consider:

 Communal riots took place in Gujarat in 2002.

 She looked for the keys in the car.

  

 ‘On’ is used when we talk about a point that is above something and is touching it, usually a flat surface. We also use it to talk about a point on road or river. For example:

 She owns a farmhouse on Jaipur highway.

 There’s dust on the floor.