First, make sure you have TWO games. Next, start a new game on one of them. 
Choose ANY starter, and then catch a random Pokemon. Trade your starter with a random Pokemon from the other game.
Then, restart your game and choose a DIFFERENT starter. Once again, catch a random Pokemon and trade your new starter to the other game.
Repeat one more time, and then catch 2 more random Pokemon. Trade them with the starters you put on the other game.
You now have all 3 starter Pokemon.