They're considered to be members of Generation 3. However, only a few Hoenn Pokemon can be found within the game and can be snagged, although others are featured as the Pokemon of your various opponents. Additionally, Kyogre and Groudon are featured on the cover, but can't be found at any point in the game (which is the only game to have this discrepancy). You can trade a Pokemon from RSE to Colosseum and it will read it just fine as well. Colosseum is heavier on Johto and Kanto Pokemon, while XD: Gale of Darkness features many more Hoenn Pokemon (as it was released after Emerald).