How do you make a certain card trick better?

Well when I did a card trick at my school’s talent show, I really messed up. I want to know how to make it better. Here’s how it’s supposed to be. First, I have a deck of cards. I tell someone to pick out three cards. Then I put them on random places on the deck. When I pull out the top three cards and they are the cards I put into random places. How I did it was that I put two of the three cards behind one of them. Behind me I have two other random cards. Behind my back, I switch the two cards with one another. They are upside down when I put them on different parts of the deck, so no one is supposed to know that there are new cards. The original first card is placed on top with the other originals. I couldn’t do it all fast enough, so I epically failed.