How often should cats be fed?

My kittens have gotten into the habit of begging for food any time someone walks too close to their food dishes. It's an absolutely heartbreaking sight. They act as if they never get fed and are starving to death. Since they are both still fairly young, I feed them wet food three times a day, and they have a constant supply of dry food. Should they be getting more? If try to give them another can of food they may take a few bites and then it will sit there until I throw it away. I'm constantly paranoid that maybe they aren't getting enough food.