A car suspension can be a complex animal, especially in how all the forces and parts interact. For example, increase the right rear spring pressure, and the left front's characteristics drastically change. In car setup, it's desirable to be able to separate the interaction betwen the components, to make setup changes less complex. With the third spring, the engineers are able to almost completely separate the ride height and pitch characteristics from other suspension forces.
Obviously, you should be able to do this with just a spring and damper on each wheel. Mechanically, it's simpler, probably cheaper, and most likely lighter. But if you can't make the setup work, if you have trouble getting the suspension to act properly, it's a good thing to be able to simplify and separate the interaction.