Most 3/DS/i games are "card1", meaning they have a non-rewritable ROM (usually a PROM nowadays, hard pressed ROM until the last turn of the decade) plus (usually) an EEPROM save chip, so they're not editable not even in theory*
* you could replace the ROM with a rewritable chip (which you most likely can't buy, because since the GBA nintendo rom chips have "smarts" in them for accelerated sequential access, hardware encryption since the DS, ...) but at that point
are you "editing the original game"?
Some others (Warioware DIY, Animal Crossing NL, Pokemon X/Y, etc) are "card2", those have everything on a single rewritable chip (though most of it is write protected), these can theoretically be reprogrammed but nobody has managed to do so (nor probably tried at all, although some the makers of some early save editor managed to completely erase one)