Si (yo/él/usted) hubiera sido rico, hubiera/habría comprado un carro.
This means that if you would have been rich at that time you would have bought the car. This can imply that the situation might have changed. For example you may no longer interested in buying the car. But if at that time the condition would have been true you would have done it. The English translation: "If I had been rich, I would have bought a car."
Si (yo/él/usted) fuera rico, compraría un carro.
This means that if at this time you were rich (you are not rich), you would buy a car. You have the intention to do it only if the condition fulfill. The English translation: "If I were rich, I would buy a car".
Si (yo/él/usted) fuera rico, hubiera/habría comprado un carro.
This means that you were not rich at that time and you are not rich now so you couldn't buy a car. The English translation: "If I were rich, I would have bought a car."
As you can see the pronoun is optional and if it can be inferred from the context you usually omit it. Also you can replace the comma with "entonces" example:
Si (yo/él/usted) fuera rico entonces compraría un carro.
It's worth noting that for conditional sentences "si" without accent must be used. On the other hand "sí" with accent is used to affirm.
You can check more on "si" in RAE.