You don't say which game you're playing, but no, you can't get the other two starters during the game (in most games your rival gets one of them anyway).
However, in some games like HeartGold/SoulSilver, you can choose one from a different set of starters, e.g. Bulbasaur/Squirtle/Charmander. But you still only get one of the three, not more.
You need to trade with others to get the rest.