There are 9 calories per gram of fat. So technically, to lose 800 g of fat, you would need to run a calorie deficit of 7200 calories.
Bear in mind that a healthy and sustainable rate of weight loss is generally 0.5 to 1 kg per week, and .8 kg in 3 days is not a healthy approach.
Also, a dramatic increase in your level of working out is likely to result in your muscles retaining water (at least in the short term). So even if you manage to burn 7200 calories, this probably won't show up as an exact 800 g loss in the scale.