How much weight could I lose in 3 months?

You're going to end up gaining weight like that because it's under 1000 calories a day. Your body will go into starvation mode causing your body to retain fat and slow your metabolism. Try to have a diet ranging between 1200-1500 calories a day. Add more fiber, protein, iron and other nutrients while taking a multivitamin too. Depending on your eating and exercise habits before the start, you can probably lose about 5-10 pounds a month.