How do you get all three starters in sapphire?

You'll need a spare game cartridge, either Ruby, Emerald or another Sapphire. You set up a new game on your Sapphire game cartridge and choose any starter. Then go out and catch any 2 other Pokemon. Now that you have three Pokemon (one of which is a starter), start a new game on the spare cartridge and choose one of the starters you don't have. Go to the Pokemon Centre on both games and trade the starter on the spare game for one of the Pokemon you caught on your Sapphire game. Save and turn off. Then start a new game on the spare cartridge and do the same for the remaining starter Pokemon you don't have.