It could that your generation 3 game is a Rom instead of an actual game. Their have been so many Hacked Roms out there that a lot of people have mistaken one for the other. Check the back of it and see if there's any dark numbers on it.
If there are it's legit if not I'm sorry, the game is hacked.
P.S. There has been cases where certain hacked rom can still interact with Gen 4 and migrate Pokemon. It had been to me that's how I know.