1. You can trade any Pokemon from Colosseum onto Ruby, Sapphire, Emerald, FireRed or LeafGreen.
2. Nascour, then Evice. Beat them and you've beaten Cipher and the credits role. Then you can trade to the GBA games in the basement of Phenac City's Pokemon Center.
3. Yes, you can make a Pokemon hold a Masterball, so you can trade it onto the GBA games.