Re: Question about catching all pokemon up through third generation
Possible, but very hard.
Firstly, you need to get Mew, Celebi, Deoxys, Ho-oh, Lugia, Jirachi from events.
If you have a GameCube you are lucky because Channel's bonus disk lets you get Jirachi. In Colosseum you can get Ho-oh but it is extremly hard. Lugia can be purified and traded to your game from XD.