From the introduction:
You have just arrived in Nuvema Town to start your adventure as a Pokémon Trainer! Professor Juniper leads you through her lab to a very special room. There, Unova's three starter Pokémon await you: Tepig, Snivy, and Oshawott.
"Which one you choose is entirely up to you," Professor Juniper says, handing you a Pokédex.
Before you decide, you use your Pokédex to learn a little about each one.