As a general rule, each unit assigned to a class translates into a minimum three hours of work per week (this is known as the Carnegie Unit). For a 3-unit class, this translates into nine hours. You are in class for 3 hours, which leaves six hours per week you should spend studying on your own. I believe that 2.5-3 outside hours per unit is a better rule...
If you are taking 12 units, you should be spending 12 hours per week in class and at least 24 hours per week studying (this is why they call it Full Time). Now, how should you be spending this valuable time?