Tricky
at; at the table, at a conference, at a quarter to ten, at your service, at a bus stop
in; in a minute, in the box, in conference, in a conference, in a bus
on; on time, on the table, on TV, on a bus
So you can see that sometimes the words are interchangeable and sometimes not.
in terms of time. At normally refers to a specific time 10:30. in refers to a gap of time so in 15 minutes and "on time" but there is no on 10:30
in terms of objects
at means besides most things but inside a very big thing. So at a door (means besides the door) but at an airport means in the airport)
in means inside
on means on-top-of
hope that helps