Not necessarily. If you get the sims 3 pets plus, it comes with the sims 3 base game. BUT if you get sims 3 pets EP, then yes you would also need to buy sims 3. I think it's better to go with the...
The Sims 3 is the base game. Meaning you need it to start off. Sims 3: Ambitions is an expansion pack. Which means it expands your base game by giving it more features, like hair, clothes, jobs, etc.