30 minutes doing cardio 3 times a week is good enough to burn those calories, but it's not exercise alone, you have to be conscience on what you eat as well. I've read that you should snack about every three hours. If you're hungry then eat a light snack. Also it's very important to eat breakfast people tend to skip breakfast and they try to make it up for lunch by eating huge proportion.
Losing weight is not an easy task. But you have to set realistic goals that you can achieve.