Yes, you can if you have a friend then trade one by one until doing it three times but it may sound long but trust me that's the only way unless you got action DS Replay.
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You can't catch a starter Pokemon. But to get all of the starters you can trade the Pokemon to another game and restart the game you were playing and get a different starter.
You can not catch any of them in black or white, but I think that you can catch all 3 in diamond.
No man no man
You start your game and get 1 starter then trade it. Then start the game again and pick a different starter. Do the same until you get all of the starters from Pokemon black and white.
No, Totodile can only be acquired by choosing it at the start or by trading. Don't listen to him. You can catch all the starter pokemon and their evolutions on Victory Road.