"In" refers to something within. Example: The peppers are IN a box.-> As pimentas são em uma caixa. "On" refers to something atop of another. Example: You're ON top of me, please get off.-> Você está em cima de mim , por favor, saia . "At" refers to something somewhere. Example: I lost the leopard... Do you know where it is AT?->Eu perdi o leopardo ... Sabe onde ele está ?

You can go to any search engine (such as Google, Bing, Yandex etc) and type in these keywords: difference in on at There are literally thousands of explanations online.