aeturos

So it's basicly the same as in the Game Boy games and the Game Boy Advance games?
A Game Boy game you can play on a Game Boy Advance, but not the other way?

Yep, exactly!

I want a gameboy game slot in my 3DS. All of my gameboy games have gone to waste.

You can buy and play Gameboy games from the Nintendo eShop on your 3DS. Alternatively, if you have a flashcart, just download Lameboy DS on it, place some Gameboy ROMs onto the SD card, and use it to play them on your 3DS!

If you still want to play your physical gameboy carts, though, you will have to buy a GBA off eBay, I guesss!