There are a few rules that I've noticed that you can follow pretty safely. In, on, and at are all used with places and time. "In" is usually used when you are talking about something being inside a boundary of some sort. "I am in America." (Boundary = Country) "It will happen in the evening." (Boundary = Late Afternoon to Midnight) "He is inside." (Boundary = Four sides of a building) "On" is usually used to describe a place you are literally on top of. It also can mean a place you are only going to be temporarily. "I'm on the train." (Temporary = Train) "Rihanna is going to be on stage!" (Temporary = On top of the stage) "I'll be there on Monday" (Temporary = the 24 hours of Monday) "Why are you calling me? I am on vacation." (Temporary = vacation). "At" is usually used to describe a single point or location. You can use "at" with general areas but for locations with official boundaries, use "in". "I'm hanging out at Frank's." (General Area) "Bob, what happened at the game?" (General Area) "I was there at 3:30PM." (Point) "The suspect is at the intersection of 9th and E Ave." (Point) "You can find more information at www.google.com." (General Area)

I am learning english but i know that you use at when you are talking about places, for example I was "AT" the park. You use ON when you say: "he"ll came ON Monday (with days, months, dates) You use IN with evening, morning, something like that. I hope you understand me :)