If you lose 300 calories a day how much pounds would you lose in 3 months?

It depends on how much you eat. A pound of fat is 3500 calories. So if you are eating the right amount then at that rate you will burn a pound of fat in about 11-12 days. Its important to note that just because the cardio machine tells you that you're burning 300 calories doesn't necessarily make it so. Your body also burns more calories as your body repairs.

Now the above assumes you are eating the right amount. If you eat too much then it will take you longer.