1. You're assuming, of course, that the 3 traded in games are published by the same publisher of the new game. Which is most likely not the case.
2. I know this is not a well-liked opinion, but it is true, how many of those new copies of the three traded games get sold after the store get's used copies in to sell? I argue that there are some, if not as many, lost sales to the publisher when a cashier says "Don't buy a new one, when we have one for $5 less."
3. What's to say that this is the 2nd or third time a game is traded in. You can no longer say the publisher has received the $48 for it.
Bottom line, there are many variables. However, most lie in the retailers favor. I'm not saying this practice needs to stop. I'm just saying there needs to be a change. This eco-system is not working.